I had a similar issue with my Rigid router. It ended up being a .75 cent part. Part number 10. below, which is a small torsion spring made of metal thinner than a paper clip. I noticed there was space between the out body and the spring and thus not supplying enough force to keep the lock lever engaged. I disassembled the screw and was able to bend the tiny spring out enough so it was supplying tension again. I also ordered spares.
I will mention when I had everything apart, I noticed that the part 6. the interlock lever piece that plunges through the router base was not flat on one side, and lightly filed the edge to make it flat where it rubs up against the router base.
I then reassembled everything and lubricated all contact surfaces with paste wax or light machine oil, and I have not had an issue with my Z axis since doing this.