Are you finishing out the level after you find them?
Personally, I havn't started my quest for skulls. (Halo live is all I've been doing) I don't have the patience right now, but I'll prolly have a couple of my friends go and get 'em for me. (and you can bet we'll cheat)
Anyway, this advice all comes from things I've read, but remember to start from the beginning of the level, and I'd suggest finishing it out afterward. If problem persists, call customer support. Those guy's are miracle workers.
