I had that happen in my 97 XJ. In my case, the plastic end button popped off because the threaded part on the inside was stripped and could not grip the metal threaded peice.
I went to the local hardware store (Ace) and purchased a tube of "All Purpose" indoor/outdoor adhesive. I filled the threaded part of the plastic cap, about halfway, with the adhesive. Then I slipped the assembly, cap and spring, on the metal threaded piece and depressed the button down. While holding the button down I used a piece of duct tape to hold the button down all the way. I let it sit like that overnight and took the tape off. The button popped back out to its normal position and have not had a problem since. Been about six months now.