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!
There are several possible issues. Firstly, you probably need to make sure that you played using the correct settings and met the requirements as needed to unlock the achievement. If you think you are doing it correctly, check an achievement site like x360a.org
or trueachievements.com to see if you can find any information about the achievement being glitched. If not, you might want to try it again. Also, you can try recovering your gamertag. This can allow achievements you have earned, but not awarded, to unlock
next time you boot up the game. Hope this helps.