Z height issues with Bosch router

Hey all
I have been using neds printed z axis kit for a few months now and it’s great. Untill today. The problem I am having is with the Bosch router getting stuck down after a plunge move. I was drilling holes in some 9mm ply with a 10mm z move and the router body didn’t spring back up with the motor. The motor mount unscrewed itself a bit from the router and then carved a 5 mm deep slot across my nicely cut sheet untill it reached the next hole and was manually stoped. I’ve tried a few times and giving the body a good bash with your fist in frustration can cause it to spring back.

Has anyone had this problem before. I’ll take the router apart soon and see if I can spot the problem.