Beat a strike with my clan, Seal Clubbers of America, last night and the rest of the fireteam ("Kevlar TheGreat" and "RowdyCookie") got the achievement where as I did not.
Edit: Solved. Thanks Tabor!
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What i have to do for all of my acheivements to unlock is go to settings» logout» switch profiles» then log back in to unlock them try it should work