The slide stop button/lever is only supposed to lock the slide back when the magazine follower reaches the top of its travel (i.e., the magazine is empty, inserted and locked into position). The mag. follower pushes on a little tang of metal inside the gun, which pushes the lever UP and causes the slide to lock back.
Try this:
Unload the pistol and the magazine completely. Close the slide. Put the safety back on just for "safety".
With NO magazine in the pistol, look closely at the slide stop lever. You should be able to move it UP by about 1/16th of an inch and it should INSTANTLY spring back DOWN when you let it go.
Now, put 1 round into a magazine and insert it. Do not rack the slide, just leave it closed. Don't load the chamber, in other words.
Look closely at the slide stop lever. You should STILL be able to move it upward by about 1/16th of an inch, and it should STILL drop back down under spring pressure instantly with a round in the magazine.
Drop the magazine, remove the round from the magazine (so that it is empty). Reinsert the magazine.
Look closely at the slide stop again. It should now be in the "UP" position all by itself (actually it's not fully up yet, but it is waiting to get there, it is butting against the bottom of the slide and being held there by the spring pressure of the magazine follower) and the only direction you should be able to push it is DOWN 1/16th of an inch. This is the position it's supposed to be in when the magazine is empty and the magazine follower engages the small tang on the internal surface of the slide stop lever.
In other words, with the slide closed:
Magazine LOADED (or Magazine OUT) -- Slide stop should move UP but not DOWN with finger pressure, and then spring the other way.
Magazine UNLOADED but IN -- Slide stop should move DOWN but not UP with finger pressure, and then spring the other way.
Whenever you insert an empty magazine into the gun (slide closed), the slide stop button should move UP as the magazine clicks home. When you eject the magazine it should instantly spring back down.
It sounds like maybe your slide stop lever is bent and/or binding in a way it's not supposed to.