A support case was created for you and you should be contacted shortly by a Customer Support Representative, if you haven't been already. You can view and update your case at the Activision Support website. After logging in, go to "My Support" then "My Cases".
What console are you playing on? It's a console based system awarding/achievement. The best I can think of for this is to contact PSN/Xbox about this or try for the same achievement again and make sure that every requirement is met.