IIRC the slide stop is retained by the spring loaded plunger in the rear end of the guide rod that engages a detent on the slide stop cross pin,
The detent is underneath and cannot be seen in this pic
Check the spring loaded pin in the slide stop is functioning as well.
Check the spring loaded pins are not sticking in both the guide rod and slide stop.
The slide stop pin should only be able to pass through the slide when the notch is aligned with the pin hole like this:
PS if you look closely at the above pic you can see the spring loaded plunger in the bottom of the hole