PGC CNC Slide on a KWA KM19 - problem

[ElCarl]

Hi guys, I'm having a problem with the new slide I have just bought and installed on my KM19.
Basically, when I rack the slide, very often it will lock back all the way, almost as if the slide lock has been activated (But it definitely isn't!).
Most of the time I can get it forward by either pulling the slide again, or hitting the back of the slide.
Does anyone know what is going on/how to fix it?
EDIT: OK, I've now found out that pushing the barrel back while the slide is locked back gets it forward again. Might help in diagnosis