Usually when the stop won't disengage during a slide rack, it's one of two things:
1. You're simply not pulling the slide FULLY to the rear.
This is easy to do with a very strong recoil spring and/or in a very small gun.
People will insist this isn't the case until they check it themselves by operating the slide SLOWLY and pulling it ALL the way back.
To test, just lock the slide open on an EMPTY gun, then, watching the stop closely pull the slide to the rear and see if the angled cut on the slide pushes the stop down.
2. The gun is fitted with some sort of recoil buffer system that is preventing the slide from moving far enough to the rear.
Again, this may only show up when you attempt to rack the slide.
Recoil buffers take up some room and do limit the amount of slide movement, often just enough to prevent or restrict the slide movement enough to cause these feed or slide stop problems.
Another less likely possibility is the slide stop notch in the slide is mis-cut and isn't pushing the stop down when the slide is pulled to the rear, or the slide stop is damaged or improperly made.
Remember, the slide stop is pushed down by the angled front of the cut in the slide and if the slide isn't pulled far enough to the rear for any reason, the stop won't be pushed down enough to release it.