I would guess the slide stop nose is too long. Bullets are hitting the slide stop nose as the slide cycles.
Remove the slide & barrel assembly.
Install the slide stop into the frame.
Insert a loaded mag into the frame.
Push a couple cartridges out of the magazine using your thumb to simulate the slide while closely watching the nose of the slide stop.
If you see interference, shorten the slide stop nose by a couple file strokes and re-test. Take care not to take too much material off or the magazine follower will not be able to trip the slide stop when the mag is empty.