Halo 3 Campaign Complete Achievement Won't Unlock

I cannot get the Halo 3 Campaign Complete achievement to unlock. I've finished the game several times on normal and heroic. Neither will unlock. Does anybody know why?

Thanks.

you have to play through the very first "arrival" level the cutscene

your just starting on sierra 117

I've had this problem with Halo as well, never mind what I do it wont give me the acheivement for the skull on the cathedral level. Never mind what skill level I do it on.

I've got it to show a number of times and picked it up but no acheivement points. Its the only Skull I dont have at the moment.