1) Check the tab on the slide stop that engadges the magazine during lockback for copper rubbings. If you see anything, you have bullet noses causing early engagement by hitting the tab early. Is your ammo too long for spec? Or, is your mag out of spec and allowing the bullets to nose forward too much? Have to tried different magazines? You can clearance the area on your slide stop with a fine file if you have experience with that but check your ammo and mag first before filing on the slide stop tab. Go slow.
2) Also check the plunger spring for sufficent tension. Beyond that, make sure there are no burrs on your slide stop plunger or inside of your plunger tube. If you have a weak plunger tube spring, replace it with a good one from Brownells, If you see burrs, remove them.
3) While you have the plunger spring and slide stop plunger out, reassemble the pistol. Place the slide stop in the gun but do not press it into the notch. Let it hang down inside of the trigger guard. Watch your frame as you don't want to scratch it. Rack the slide a few times. If the slide stop swings wildly and with sufficent tension, you may have to refit your barrel link and barrel feet. This is a bad case and I hope this isn't your problem. It usually only crops up on tight fitted target or custom guns.
4) If your slide stop is not dimpled to hold it down propertly, you can dimple it. But be careful! If you don't dimple it in the right place or with the right depth you can ruin your slide stop. You don't always need the dimple but it helps out.
5) If you shoot left handed and the slide is locking back on the dissassembly notch, you need to have your slide stop set up for lefty shooting by properly fitting the retaining tab and perhaps cutting the slide stop pin flush with your frame.