When you say: "With the PCR slide stop installed" are you referring to the OEM Slide Stop from your PCR, or the "PCR" Slide Stop that came with the KADET Kit ?
If it's the Stop that comes with the Kit, try using the OEM Slide Stop from your PCR, not the "PCR" Stop from the Kit, see if that helps
When mounting my KADET on my two Lowers, I always use the Slide Stop that came with that particular Pistol
Hasn't failed me yet
When you say: "the slide locks back on an empty magazine but it is catching on the mag" is the Slide catching on the Follower or the Mag Body / Feed Lips ?
In either case, that shouldn't happen