Halo 3 skull question?

[yoitsmenigel]

I have almost all the skulls, but I'm missing the catch skull and the cowbell skull. now, I know how to get both of them, its just, they won't appear. I'm not doing anything wrong and I'm doing exactly what every person I've seen do. why won't they appear?!

 

[oberle10]

always make sure you start at the beginning of the level and at its not on medium. for the catch skull make sure you arent destroying the huge tanks in the area cus it goes away. and on the cowbell....i dunno just make sure you get all the way to the top cubby