I know this game is old, but I too had the achievements not unlocking issue. After trying everything I could think of (clear cache, delete & recover profile, delete & re-download game, delete gamesave, clear last dashboard update), the magic thing that did it for me was to CHANGE MY GAMER PICTURE. I cannot explain how or why, but after 1.5hrs with MS support, he suggested it. I played the 1st arena, didn't die and pop. I called my buddy, he signed on and we did the 2 remaining arenas and I got the co-op and clear all arena ach's. Absolutely bizarre to me, but it did work. Hope this helps someone else.