Skulls arent there

I have been playing halo ce for a while now and have found half of the skulls giving me the achievement but i cant seem to find anymore… i looked on youtube and i found out were they are and when i go to these places they are not there… im not on easy on accident and ive tried every different way i can imagen… help!!!

Are you playing on Modern Graphics?

You need to be for the skulls to appear.

Well…maybe the videos you watched weren’t accurate. Or you already have those skulls. Or you have more than you think.