This also sounds like a mornic question, but are you sure you got the right skull? there's 2 on that level, and the one you get within 5 minutes is just the blind skull, so it won't give you an achievement. Iron is near the end of the level on the platform above when johnson is held.
This sounds like a moronic question but, have you looked in your achievements to see if its there, because sometimes I'd have an achievement that didn't unlock. But then I'd look in my achievements later on and it would randomly be there.
Yea, I'm going with "Gayest Glitch Ever." Every time that I mentioned playing through and getting the skull, I completed the level. Plus, I went to the second level and found the skull, and the achievement unlocked immediately after I picked it up.
I need help! My friend and I were playing co-op, and he showed me where this skull was in the first level (on Legendary). I got it, but it did not give me the achievement. I tried going through solo (also on Legendary, then on Normal), and it still did not give me the achievement. The game registered that I have the skull, because I can set it on the Meta point mode, but it won't unlock the achievement. My friend and I are both baffled, and have no idea what's wrong. Help?