on halo 3 i cant get the skull on the level the storm?

[Jacko]

for some reason on Halo 3 i cant get the skull and yes i dont destroy the wraiths but when i get up there its not there for some reason. please someone explain why!!! and it is also on normal

 

[Hatred behind the eyes]

It's like the IWHBYD skull on halo 2, Its diffucult to explain basically you have to do every thing perfect it's not a glitch or anything keep retrying the level if it doesnt work never revert back a checkpoint as this does not work but it is a real skull.