I’m too lazy to see if anyone has mentioned this past the first few posts. I’ve reached Terminal 7 on The Library and the achievement doesn’t pop. The Terminals leading up to it have all worked, and I’ve completed Zombie Repellant since. I also have five achievements that have been listed as “Unlocking…” since day one (Terminal 7 isn’t one of them). The “Unlocking” ones have been completed, but it seems to have gotten stuck mid-alert.
If it says “Unlocking” it means you have unlocked it, it’s just waiting to confirm it from their server side.
Hehe. In the middle of playing FIFA 15, I unlocked the Heaps of Medals achievement for Halo. WTF.
I keep hearing about delayed achievements but I guess it varies from person to person.
I’ve had four stuck at unlocking since day one, and I have know idea when/if they’ll ever actually pop.
I’ve had 6 unlocking since day one 
Hope it’s sometime soon.