Perhaps the spring loaded plunger in the guide rod is not exerting enough pressure on the detent notch in the slide stop pin? It the image of the end of the guide rod, the spring loaded pin doesn't seem to be protruding as far out as it should.

Check the spring under the plunger inside the guide rod for damage (or oxidation), the condition of the end of the plunger itself, and the condition of the detent slot in the slide stop pin.

And, is it possible for you to move the slide stop out of position at any other location on the slide other than when it is in the proper alignment for disassembly? Perhaps excess gauge between the slide and the frame rails (either up and down or side to side) is allowing the slide stop to move over out of position during cycling?

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Ret. LE, FA Instr, S&W Armorer