Maybe:Hotter ammo,firmer hold,lube,any rubber buffers added?.45 slide stop in a 9mm.
Q:Is it real easy to engage the slide stop manually?Smetimes a little detent is drilled in the slide stop,and it can be too much detent real easy.
Empty mag inserted,pull the slide full back,does it engage?
Weak mag spring?Try a new Wolfe +5%