A lot of people have reported this problem. It seems to be a known bug that affects some people. I didn't experience it but I've seen numerous posts asking about it. If restarting the level or rebooting your 360 doesn't fix it, probably creating a new game is proabably your last resort. You could also reformat the 360 drive but I doubt that would help.
Wish I could be more help but I haven't seen a definite fix for it yet.
Or you could wait for a patch...
Good luck with it