We all know that GOG struggles to secure AAA titles from big publishers because of their stubbornness in implementing DRM and because of the vastly smaller user base than Steam.
But what I don't see mentioned often is that even indie titles released by mid-sized publishers seem to be starting to abandon GOG, or at least leave it as a last option to scrape together some extra money.

You can also see how indie developers who have been lucky enough to be successful have almost immediately ditched GOG and stopped releasing their games on this platform, which then lacks a lot of sequels to successful indie games. Here are a few examples:

Spelunky: released on day 1 on GOG in 2013 by Mossmouth.
Spelunky 2: released only on Steam in 2020 also by Mossmouth.

Risk of Rain: released 3 months late on GOG in 2014 by Hopoo Games.
Risk of Rain 2: released only on Steam and EGS in 2020 also by Hopoo Games, later acquired by Gearbox.

Octodad: Dadliest Catch: released on day 1 on GOG in 2014 by Young Horses.
Bugsnax: the developers' new game released on EGS in 2020 and Steam in 2022 and they said they were not interested in GOG.

Q.U.B.E. and Q.U.B.E. 2: released almost at day 1 on GOG in 2014 and 2018.
Q.U.B.E. 10th Anniversary: released only on Steam and EGS in 2022 and the developers said they were not interested in GOG.

Slime Rancher: released on GOG in 2016 by Monomi Park. They never released the DLCs of the game.
Slime Rancher 2: released only on Steam and EGS in 2022 and the developers said they were not interested in GOG.

Backbone: released on day 1 on GOG in 2021 by Raw Fury.
Tails: The Backbone Preludes: released only on Steam and EGS in 2023 and no sign of it coming to GOG.

Darkest Dungeon: released on day 1 on GOG in 2016 by Red Hook Studios.
Darkest Dungeon II: it has just been officially announced that the sequel will be released on Steam and EGS. No mention of GOG.

Supergiant Games used to release its games on GOG about a year late, Hades was released on Steam in 2020 after the exclusive deal with EGS.

Devolver Digital delayed the release of Return to Monkey Island on GOG specifically ignoring any request for explanation. Now it seems to have cancelled every release on GOG.
Notable missing titles:
Wizard with a Gun
Reigns: Three Kingdoms
Cricket Through the Ages
Children of the Sun
Reigns Beyond
The Crush House
Neva

The Talos Principle 2 (Delayed)
The Plucky Squire (Delayed)

Humble Games used to release its games on GOG on day 1, now Midnight Fight Express was released with considerable delay, and SIGNALIS doesn't look like it will ever arrive here. Now they have stopped releasing games on GOG.
Notable missing titles:
Protodroid DeLTA
Mineko's Night Market
Coral Island
While the Iron's Hot
Rolling Hills
#BLUD
Bō: Path of the Teal Lotus
Wizard of Legend 2

-Soon-
Monaco 2
Never Alone 2
Rolling Hills

Raw Fury used to release its games on GOG on day 1, but now it has completely stopped releasing them here and won't answer any questions about them. On December 13, 2023, they resumed releasing some games on GOG.
Notable missing titles:
Dream Cycle
Superfuse
Tails: The Backbone Preludes
GUN JAM
Mr. Sun's Hatbox
Cassette Beasts
Kingdom Eighties

WayForward used to release its games on GOG after a few months, but it seems to have abandoned GOG completely since River City Girls Zero.
Notable missing titles:
Spidersaurs
River City Girls Zero
RWBY: Arrowfell
River City Girls 2
LUNARK

Annapurna Interactive has long since stopped releasing games on GOG.
Notable missing titles:
Stray
Neon White
The Artful Escape
Twelve Minutes
Outer Wilds
Solar Ash
Storyteller
COCOON
Thirsty Suitors

PLAYISM used to release its games on GOG, for a few years now, it has completely stopped releasing any games.
Notable missing titles:
Mighty Goose
The Good Life
GNOSIA
Momodora: Moonlit Farewell
And many others...

GOG remains attractive to self-publishing indie starters and small publishers, who benefit from every bit of additional media exposure, as well as having a strong loyal group of quality publishers such as Akupara, Wadjet Eye Games, and THQ Nordic, not counting various publishers of adult games that found a home after experiencing Steam's instability.

Nonetheless, GOG is increasingly unable to secure the release of successful indie sequels, and the users are forced to buy the aforementioned on Steam or EGS, or wait years in the faint hope that they will finally arrive on GOG.

In your opinion, what are the issues that lead even indies to abandon GOG at the first sign of success, and what could be done to convince the developers to stay on GOG?

The Epic Games Store happened. This is one of the results. You can read or hear that developers starting to have problems to deal with several versions they have to release with each store. Steam and EGS are two versions. With GOG they have three versions. This is already too much for smaller studios. You also have developers here which removed games on GOG because of low sales. In this case they don't want to do the extra work for a GOG version anymore, e.g. CHANGE: A Homeless Survival.

The most enticing answer to that, from developers' point of view, would be for GOG substantially to lower the financial cut that it takes from the sales of their games.

But whether GOG could afford to do that is another matter. Maybe they can't.

What are the issues that leave indies to abandon GOG: probably the main thing is money, as in they don't make enough of it from GOG to make them happy with having released their games on GOG.

But there's probably no easy answer as to how to fix that problem.

Other than just giving the devs more money, another thing that could theoretically help would be if GOG employees flied to the devs' cities and schmoozed with the business dinners and things like that, but that probably wouldn't make financial sense for GOG to do in regards to indie games, even though it would increase the devs' favorability opinions in regards to GOG. That strategy probably would work in regards to some AAA devs who historically and currently shun GOG, though.

As for calling THQ Nordic a "strong loyal publisher" on GOG, I'd say that's only half-true, and the other half of it is false. Yes they release their games here on day 1, which is good, but on the other hand, many of their games are missing Acheivements, or missing patches, or having GOG-version-specific problems that they never bother to fix no matter how much time passes, etc. etc.

So even the "good loyal" publishers are also, simultaneously, bad publishers too when it comes to GOG, at least in THQ Nordic's case (and also in Larian's case, who also, like THQ Nordic, release their games here on day 1 on a loyal basis...but yet who left a game-breaking bug in the GOG version of D:OS 1 in the game for 5 years from the time they acknowledged knowing the bug existed until they actually finally bothered to fix it with a patch 5 years later).

It's very sad to see that EGS, which in words was hated by everyone, has managed to erode in a very short time the small space that GOG had carved out for itself in more than a decade. I can only hope that GOG can find a way to survive in the long run.

It's not necessarily so that those indie developers are abandoning GOG, in many cases it has been GOG who rejected them.

There used to be a long period when almost every indie game I was interested in made an attempt to be on GOG, but they got every time the very same refusal message.

And at least one indie developer who has a game on GOG told before that GOG didn't accept any of the following games, including direct sequels to the one already here.

So it makes you wonder how often it is the store and how often the developers who choose not to co-operate. It has undoubtedly changed things. In all fairness though, one indie developer whose games are on GOG (without any apparent problems, I might add) told publicly that Epic didn't even bother to respond when he asked about the possibility to have his games on that platform.

So it makes you wonder even more if there is some method to these stores' madness, or if they are just trying to fill up some accepted/rejected numbers on some spreadsheet.

That's precisely one of the things I asked for in that survey they did, more high-profile indies and their sequels, I understand not getting more triple A games due to the DRM-free policy (and honestly I couldn't care less, nowadays the vast majority are soulless husks) but GOG has to secure every noteworthy indie they can at least.
And I have no idea why it happens, sure I get delaying the game a few months to patch it up and focus on a single platform, but when even the games that are DONE (like Hades) don't arrive on GOG... that's a bad sign, either GOG is not reaching out to the devs/publisher, or the money to be gained is just insignificant.

They have lots of indie titles as part of their weekly giveaways:
https://www.pcgamer.com/epic-games-store-free-games-list/

If you look closely to the list you'll see Supraland. This was another case where a developer removed the GOG version here because of low sales and the extra work. In this case the amount of money to offer the game for free on EGS for a week must have convinced Supra Games to release an EGS version.

too little sales to make it worthwhile

I don't know for sure how big an impact EGS has had, but this is exactly what many of us warned of a couple of years ago in the 'GOG teams up with Epic' thread - start selling Epic versions of games via Galaxy, and publishers will start to question "why bother with a 'real' GOG version if it means 3 vs 2 builds to support plus a potential 70% (GOG) vs 88% (EGS) cut of what does get sold outside of Steam"...

The other non-Epic specific issue is "Support Fatigue". Assuming they publish on console too = build, support & update 3x versions (Sony / MS / Nintendo or Apple / Android / Switch) for 3x platforms, but then also have to do another 3-7x versions (GOG, Steam, Epic, Origin, UPlay, Humble, itch.io, etc) just for one PC platform, ie, make 6-10x builds for 4x platforms? Many of them then start to gradually drift towards capping supported PC stores to just 1-2 (especially if the game that brought them initial success needed a ton of patching and they have sour memories of having to build and upload 6-10x different builds of 30x patches each to 6-10x stores). And these days #2 is Epic rather than GOG in terms of being able and willing to simply "brute-force" throwing endless money at publishers.

Thanks for bringing this up!

Agree 100%... and have posted similar on this subject.

IMHO this is the biggest barometer of the health of GOG... and sadly it doesn't look good when small companies (and individuals) drop GOG for their follow-up releases.

I don't have time to expand on this ATM, but will try to post again later with more thoughts.

GOG is terrible at extending their bridges. From the manual invoices, to the terrible review system, to the often non-existent community, to the abject indifference towards Linux, non-existent mod support, as far as I understand submitting games on GOG is very poorly described and the build system is completely asinine, and how developers don't seem to have a real voice on GOG, I'm not surprised that indie devs are choosing Itch and other platforms over GOG.

And then the store gets flooded with cheap porn games of which many are using just the barest modicum above the bog standard RPG maker toolkit, and I'd think of somewhere else to shop not because I'm a prude but because the bar for quality fell though the floor.

Also, indies tend to care about social issues, and GOG's response/lack of moderation towards many of the users who have objected, balked, or gotten threads closed in the past? That can't be appealing either. (Thanks jerks, who got the LGBT+ thread closed.)

It sounds more like personal complains than looking at the real issue.

Agreeing with others

Epic has been a big change with many Pubs choosing that as "the alternative to Steam" instead of GOG. Their lower cut does make a huge difference to small studios.

Now IIRC when GOG ditched Regional Price rebates they claimed it was so they can pay Devs more. I assumed it meant lowering GOG's cut, but that doesn't seemed to have happened.

And the other big Issue is communication.

Across every aspect of the business, be it with consumers like us or commercial groups like indies, GOG lacks any coherence. Curation is Opaque, with no logic on what gets selected, which titles get "In Dev" or "Coming Soon" or a stealth day 1 release.

I've said this before, GOG needs a system of "Trusted Developer", that allows Publishers or Studios with a proven track record to bypass curation and have more control over release and communication.

This 100%. I have seen developers talk about how it's too much work to release games on multiple storefronts. That there happy with just steam or even Epic. The games you listed probably, did not sell well on GOG compared to steam. So the developers see no point releasing the games on day one for GOG.

It is a rather concerning trend, especially when GOG-friendly developers who have multiple existing games here refuse to release multiple new ones, eg, no Oxygen Not Included and Griftlands by Klei (vs Don't Starve, Invisible Inc & Mark Of The Ninja). Was there some falling out? As others have said it's probably a combination of 1. Epic Games have ended up competing against GOG just as much as Steam and 2. 'Store Overload Support Fatigue'. It's almost like everyone who demanded every store (even small ones) have their own client, their own achievements, etc, and force developers to support the lot is now seeing the obvious 'fruits' of that - The Steam Model (forcing developers to "write for the store" rather than the publisher) has only ever really benefited Steam (but harmed smaller / multiple stores) by design...