I tried getting this achievement several months ago but I never got around to asking for a solution to this problem until now, anyways, I collected all of the skulls in the game but the ach didn't unlock, then I looked at my skulls list and noticed that the skull on the library level (I forget what the skull is called) wasn't unlocked but I knew I picked it up, then I went back to that level and went to the location of the skull and it wasn't there. Is there a way I can fix this?