I collected all the Halo 2 skulls in legendary, got all 15 achievements for each one but “Trophy collector” did not unlock, the achievement progress shows 14/15 for some reason.
That means that the achievement didn’t register one of the skulls (not to state the obvious, lol). What system are you playing on? If it’s Xbox then I suggest hatd resetting your console to see if that solves the issue. If not, I think your only other option is to collect the skulls again.