The T3 Wolvers seem to enjoy teleporting so much, they defy the laws of gravity to do it. I noticed a couple of times after me or my group shot at them, they would sometimes choose to teleport outside of the depth. From there they kinda stopped moving/had limited movement. Making them teleport seemed to be the one way to get them back closer to us.
I took a picture.
http://cloud.steampowered.com/ugc/612721425819612758/DD845AFC1E3BF0C4CA0...
Monsters sometimes manages to go off the stage. I have seen this happen when the wheels push slags off the edge in FSC. My best suggestion would be to submit an in-game bug report with F2 the next time this happens to you or anyone else that is reading this. This sends the issue and other useful information for the client itself over for the team to look at.