Why don't the skulls appear in Halo 3 sometimes?

[Andy]

I watched the video guide on youtube so i could get the skulls, but it's not there when I get to the spot in my game. Did I do something wrong? I'm playing it on Legendary so it should be there, but it is not.

 

[Chance W]

It depends to get the gold skulls you have to go through the entire level and then get them and sometimes if you do something to far in the story they will dissapear but not forever