I've done some testing and figured out what the/a problem might be: Indirect kills don't count. So if you use the Atomizer to shoot one of the targets up in the air, and he dies from the fall, it won't count as a kill. If you use a sticky bomb to set the floor on fire, and the target dies from the fire, it doesn't count. If you use a vehicle to push another vehicle on top of a target to kill him, it doesn't count. The problem is then that they still die, so you can't complete the mission.