My compact grip for the P320 came in.
Removed trigger group from full size, replaced mag release and inserted into the new grip.
Placed the slide on the grip and locked the take down knob without locking the slide back first.
Work the slide a few times and go to dry fire and nothing.

Swapped internals back to the full size without locking the slide back first and same issue.

Long story short, as long as I lock the slide back with the mag release BEFORE turning the take down knob all is good.

Granted I didn't read the manual so not sure if it is discussed but I find it very strange.
Just a heads up in case it happens to anyone else.

The slide lock resets the safety that keeps you from rotating the take down lever with a magazine in.  Take the fire control out of the grip module.  Lift up on the slide stop and you will see the bar along the left side of the magwell jump forward.  Always lock the slide to the rear with the slide stop when you reassemble. David

Always RTFM https://www.sigsauer.com/wp-content/uploads/2016/07/P320.pdf Page 42 step 6.