I've searched the forums to an answer to this question and have found a few solutions but none of which have worked. Basically, I beat Assassin's Creed Brotherhood offline and have about a 90% sync (completion) of that game and racked up most of the achievements that go along with that. Since then I've played Revelations and looked back for my Brotherhood achievements to find that there were none. Oddly enough after I finished playing with Revelations some of the achievements I got in that game are also missing. I do play my games on a few different consoles and my achievements follow me every time I recover my GT to the best of my knowledge.
For the Brotherhood achievements that I'm missing I've tried:
1. Downloading my GT again to my hard drive.
2. Signing out of xbox live and signing in again.
3. Getting one of the achievements I have gotten in the past again, waiting a few hours to see if the rest popped up.
My game is still saved to the hard drive at 90% sync so its just a bit frustrating to me that I only hold 1 out of 60 achievements in this game.
Hope someone can help.