In Halo 3 there is a skul in front of me and yet I cannot pick it up. Why?

[Zero]

I keep going up to it but I can't pick it up. It is the grunt birthday party skull in crow's nest. I already have the other skull in crow's nest, but that is the only skull i have.

 

[Xboxgamer]

You have to hold RB (Right Bumper) but if you were Than just try the level over again because its not hard to get there

 

[thexbox360player]

its probably a glitch try restrt the level from the beginning