It's the Xbox gamer's worst nightmare -- you do everything that you need to pop an achievement but you didn't get the "achievement unlocked" pop-up. When you check the progress on your console, it says it's "Done! Unlocking...". The dreaded Done! Unlocking... achievement
We've heard stories of achievements in this state (or stuck at 100% completion) for up to six months before finally popping. This is an annoyance at best, but at worst it could cost you your place in a contest such as The Ultimate Head to Head Contest 2.
We have some good news for you -- the Xbox support thread on this issue has been updated to read:
If you were connected to Xbox Live and you completed the full requirements for the achievement but the achievement seems ‘stuck’ at 100% complete or “Done! Unlocking…”, go to the achievement detail view on Xbox.com or on your Xbox One console to double-check whether it is still locked.We also reached out to our contacts at Xbox and have confirmation that work has been done behind the scenes so that the stuck achievement should be forced through when you view it.
To make things easier for you, we've added a new panel to the Xbox One achievement pages:
New TA panel makes things easy
This includes a link to the appropriate page on Xbox.com for that exact achievement. Simply log in to your Xbox account (if needed) and it should force the stuck achievement through the system.