Upon release of the Master Chief Collection, a friend and I began the campaign on legendary. A few hours into the campaign, I noticed I wasn't receiving achievement notifications at the end of missions. I snap the achievement app and receive "Please check back in a while: You haven't lost any achievements... we just can't show them right now. (0x87c47500)." When achievements actually started working for me again, all the achievements (I had completed but did not unlock) had the completion bar filled, and said "Done! Unlocking..."

That happened on Tuesday, as I post this it is Sunday. On the support page it said wait "a couple of days." Its been five. No unlocks. Nothing.

This honestly wouldn't bother me as much as it should, except it is preventing me from earning the in-game unlocks paired to the achievement, and it is preventing me from reaching completion.

And yes, I've replayed some of the missions and completed the requirements for some of the achievements again, however I still haven't unlocked any of those that say "Done! Unlocking..."

TL;DR Completed multiple achievements but didn't actually unlock them. Get the message "Done! Unlocking..." but it has been like that for five days.