It's probably not the slide stop, but the magazines. I would field strip the gun, then insert the slide stop and move it up and down. It it moves easily, it's the magazines. If it's hard to move then it's the slide stop. Sometimes they can be fixed by polishing the arm. However the slide stop is a part that rarely wears out, I'd bet on the mags needing new springs.
I would also blame a bad mag. The spring being weak is the main cause. Have you ever had feed problems? It could be the follower getting stuck also. If cleaning the mags really well doesn't solve your problem, I'd replace the springs.
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