Halo Reach Achiev unlocked 3 of 12 missions ? ^^

Halo Reach. The Achievements really have a Problem, Completed the game on Normal, now i see that only the Achievements from missions 5,6 and 12 unlocked…
And no i never have any skull active…
Maybe problem with the Internet before ?
Possible to Unlock it, without playing everything again ?

Ingame it shows everything Complete

What platform are you playing on? If you are on Xbox then there is a chance the achievements will unlock randomly. However, I would recommend running through the missions again.

i m on PC, on Steam the Halo: The Master Chief Collection