Remember that you only get achievements from killing active players or Elite AI's as well as only in ranked games. If you believe that you should have gotten an award you can always put in a support ticket and a Game Operator will take a look and let you know what happened. I can tell you from my experience it is because of one of the three things I mentioned above.
Hope this has answered your question if you have further question(s) or concern(s) please post them.
