Ok im up to the famine skull on the ark (on normal) but there is no skull on the fourth arm (where the skull is). I even tried not killing any ghosts or the wraith but it wont appear. I tried to beat the tanks and our warthogs and w.e. so i tried the next skulls after that level but none of em are showing up where they should be after doing everything perfect.does this always happen on some discs or what.