Did you break one, go to dashboard/turn off xbox, then return to the game and break the others and so on? I've heard the achievement glitches unless you do them all in one session.
Deus Ex Achievement Guide | xbox.com
Retail Completions: 38 | Arcade Completions: 15
Seriously 3.0: Total Onyx Medals - 24/65 | Level 86
Currently Playing: Assassin's Creed III, Borderlands 2