This is quite a long winded problem and request but I'll try my best to explain it well enough.
I've been playing Dark Souls on Games for Windows but after earning a couple of achievements, the achievements have stopped counting towards my Gamerscore on other devices such as the Xbox website, Xbox 360 and Xbox One.
My Gamerscore is only updated on the Games for Windows interface accessed while playing the game, meaning the regular Games for Windows computer application/program still states my old Gamerscore.
On the majority of devices my Gamerscore reads 77969, saying I have only unlocked 2 achievements for Dark Souls. While on the Games for Windows interface while playing Dark Souls reads 77984 with 3 achievements unlocked. Meaning the the achievements I'm unlocking on Dark Souls are only contributing to a Gamerscore only visible to myself, only while playing the game.
I contacted customer support and they sent me to the forums to get in contact with the Enforcement Team. Hopefully you'll be able to resolve my problem and have my achievements show to on all devices.