Achievements not unlocking

So ive been trying to complete all the campaigns on legendary in order to get the Helioskril armor for Halo 5, but ive noticed that as i play achievements arent unlocking, The first one that didnt unlock after beating was Reqium on Halo 4. I just finished beating the prophet of regret on halo 2 and also no achievement. These missions had no skulls on or anything. Any help 343?

Not sure if this will help or not but I’ll tell you what happened to me in MCC and Achievements.

I completed Halo 4 on Heroic just prior to Halo 5’s launch to refresh myself on the story. Achievements popped all the way through for most of the levels and I didn’t notice 2 didn’t until I completed it and no achievements for completing it. This was despite every level showing a completed at Heroic status when browsing the level list.

When I looked back at my achievements I saw the 2 for Requiem and Shutdown hadn’t unlocked. Those were also the only levels in Halo 4 that I didn’t complete in one sitting but saved to come back to (I’m not sure if that’s relevant or not but I remember it happening to me on the 360 version too).

Anyway, I completed Requiem and got the achievement. The next day I completed Shutdown and no achievement. What on earth? I then opened Halo 3, played a bit and no achievements popped there either! Basically, no more achievements unlocked after that Shutdown one was stuck.

I turned my Xbox One off, turned it back on and a ton of achievements popped all at once (without needing to load a game).

On Halo 5 I had something similar with the achievement to complete Mission 13 with the Scorpion still intact. It was in the Done Unlocking… state and even a restart didn’t force it to pop. I came back a day later and it had unlocked (but no achievement notice popped) with the correct timestamp.

Hopefully the same will happen to you.