Diagnosis
The molded plastic threads in the tool's housing are stripped. This is common if the clip was overtightened or impacted.
How to Fix It
Get a Milwaukee replacement screw (part # 49-06-2001). It is longer than the original. It will bite into fresh, deeper plastic. Do not use a standard hardware store screw. Coat the new screw's threads with a tiny amount of epoxy or thread-locking compound before installing. Tighten only until snug.
If That Didn't Work
You must install a threaded insert. This requires drilling out the old hole and installing a metal insert. It is easy to crack the housing if you are not experienced.
Heads Up
Do not use a larger diameter screw. You will crack the housing boss and the repair becomes much more difficult.