This happens whenever there are too many enemies in an area to all fit on the screen; so the game simply does not show them nor allow you to kill them. (kotaku.com)
In the standard Capture game mode, teams earn points by completing tasks such as destroying enemies in an area, or more interestingly, by capturing objectives. (impulsegamer.com)
There is also a lock - on: pressing the left bumper button will rotate the camera to centre on the largest enemy in the area, which does make combat substantially easier. (darkstation.com)