So If you ever played Gears of War 3 and you unlock an achievement and it pop's up but when you go check your achievements whilst playing it says you didn't unlock it I have your solution. This happened to me while I was playing I couldn't get achievements until now. I was messing around with my profile when I signed out I tried signing back in it said it was corrupted. So I turned off my XBOX and turned it back on. When I tried signing in it mentioned something about the membership and that I should recover my account. So when I recovered it I hopped back on Gears of War 3 and BAM I unlocked an achievement and It counted. It showed that I did in fact unlock it. Try what I did out and tell me if it works for you!