why are the halo 3 skulls not showing up?

[Randy G]

i am wondering why the rest are not showing up i have 8 skulls but when i go back to any level to get the ones i dont have there gone and i already beat the game

 

[aaronlealaiauloto]

It must be on any difficulty above easy and you must start at the beggining of the level and not rally points.
GamerTag:
Lealaiauloto