I've sometimes found a guide rod plunger with a peened/rolled burr on the top edge, or a slide stop lever pin with a burr within the scalloped recess that holds the plunger head. Lightly dressing the burred spot, and a very small drop of oil (or grease) on the lever's pin, usually served to resolve the issues.
Of course,
many more times it's usually been caused by the owner/user not properly aligning the slide & frame notches for lever removal, or letting the slide/frame alignment shift while they were trying to remove the slide stop lever, and then no amount of force was going to work.