I guess I'm just being dense (again) this afternoon... But, it seems to me that having the slide stop stay UP would make it more likely to catch the slide and stop it, not less likely!
When you RELEASE the slide, you press down on the slide stop.
When the mag follower comes up the mag tube, the spring in the mag is pressing upwards on the follower so that the front of the follower presses on the underside of the slide stop, and pushes it UP, catching the slide and locking it open.
What am I misunderstanding?
For it to work properly, it seems to me, the spring tension has to be such that it keeps it in position JUST BELOW the height needed to lock back the slide, and not be so heavy that the follower, pushing up, can't make it move up and lock the slide the next time it passes.
(I've missed so many obvious things these past few weeks, I won't be surprised to have missed another. I find this analysis, like UT83's suggestion that a weak recoil spring is a possible cause of premature slide lock, beyond my understanding.
Please note: that does NOT mean either of you are [wrong], but only that I don't understand...)