I think armorer951 nailed it in post #6.

The spring loaded plunger in the rear of your guide rod is what holds the slide stop in place and yours is binding and not protruding far enough.

Grab it with a pair of pliers and pull it out, lube it, re-insert it in the end of the guide rod and make sure it sticks out past the first ledge of the plunger and is springy.

John