Why wont the halo skulls show up?

I have the difficulty set to normal, and no matter what I do, the skulls for the last 4 levels(The ark, The Covenant, Cortana, and The last level) they wont show up. I hve tried all the edited settings on the skull page for the scoring for none, team, and free for all, yet only the ones that I have found are there. I have also tried starting from the begining of the levels like most sites say to, but they still don't appear, please help!