I’ve beaten every halo 4 mission but it says a few are missing (legendary)
Were you connected to Xbox Live? The game will only track your progression if you’re connected.
Exactly. If you didn’t beat all the levels while connected to Xbox live you won’t get the commendations which is what unlocks the armor. (If that’s what you’re going for.) The achievements don’t get you the armor. You also have to start the levels from the beginning.