Why don't I get the achievement for the Iron Skull in Halo 3?

[Chad A]

I started the first level on normal, from the beginning, no other skulls turned on, found the iron skull, picked it up, walked around and limbo -ed for a little while...... no achievement. I know i did everything correctly... to my knowledge that is... Anyone else have the same problem?
yeah, i did. i met the first gold brute. killed him, blitzed through the rest, and ran up to the rock with the skull, sitting right there.
yeah, thanks. I'll definitely look into some of those possibilities. thanks guys.