I don't know what it is, but GOG Galaxy has a terrible time recognizing if achievements are unlocked or not. Maybe it's the game since some games unlock them really well, while others don't do it after so many hours of grinding towards them.

After beating Bioshock Infinite Clash in the Clouds with all the gallery items purchased, and beating the 1999 mode without purchasing anything from Dollar Bill, the achievements remain locked. I played through the fucking 1999 mode and now it's wasted hours. WTF

Same thing with the damn XCOM game. The secret achievements is supposed to be beating a certain mission with at least 2 turns remaining or something and it's still locked away after beating the mission with 4 turns remaining. I so wanted to just uninstall the game after getting it but I still am not sure if it actually is the secret achievements or not.

I find beating a game with 100% achievements unlocked to be the most satisfying but so frustrating when they don't work due to non-player issues. I hope GOG will improve this in the future.

I imagine that could be quite frustrating indeed. I'm not big on achievements myself but I know it is an important thing for a lot of others. A number of achievements I accomplished in Witcher 3 did not register in Galaxy ever as well. I wasn't actually trying to achieve them mind you, but rather just noticed that I had accomplished certain things and did not get the associated achievement. GOG/CDPR fixed some of these issues over time and some of them granted eventually but others still have never granted.

Hopefully they can work the rest of the kinks out some day, however it may be bugs in a game that causes it too in the case of 3rd party games. May require a publisher patch to fix. Hard to say.

Ugh.. that's infuriating.
A lot of people like achievements, so Gog should really fix these issues.

Check this thread and report the broken achievs there, please.

Hmph... you think that's bad you should have seen the steam achievements back in the day they just WOULD NOT UNLOCK!