Tracker device not working properly?

Hi there. I am trying to make a zombie wave map however I am having a problem with my trackers. The way this map works is the zombies come in waves that get bigger and bigger overtime. The waves end when the players have killed a certain amount of zombies. However It seems like the tracker device is miscounting some kills. I get to the point where my tracker says there a few zombies left to kill. The number of zombies is different every time and can happen on different waves. The tracker is set up to start the next wave when the tracker is complete however the tracker cant be completed because I think it is miscounting. The only other thing that could be the problem is the spawners however I have a feeling its more of a tracker issue.