We're excited to announce that we've fully migrated the Xbox Forums to their new home, here on the Microsoft Community forums!
As of today, going to forums.xbox.com will redirect you here to the Xbox area of the Community forums. We've moved all of your existing threads over so you can continue and start conversations with the new forums experience.
We sincerely appreciate the feedback you have provided over this transition and can't wait for you to check out the new Xbox Forums!
That happened to me, as well. I just kept playing more Spartan Ops and it unlocked eventually. I don't know exactly what I did to unlock it, but I remember that it did not unlock right after I finished all of the Spartan Ops missions.
Also, the achievement that you get for defending the generators on chapter 1 of episode 5 is also glitched.