I bought an FCU from a sig 365 from a standard compact unit.  I have the grip module as well.  I bought a brand new 365xl slide assembly complete from Sig Sauer.  I installed the FCU in a Wilson Combat grip module, no issues, attached the slide and the trigger will not fire.  If I charge the gun, empty and pull the trigger it's mush.  If I wiggle the slide I can hear the click of it firing.  I can then charge the slide again, let the trigger out and hear the reset.   If I pull the trigger again mush.  I can post a video if that helps diagnose it.

Appreciate any help from the gallery.

Thanks.

I took it apart and assembled it again and have it working.  I'm going to replace all of the springs in the FCU and wait on a Gray guns trigger.

Glad to hear you got it working
Did you figure out what was causing the issue? I was going to suggest there might be some mismatch between the fcu and the striker, if the tolerances aren't tight enough it could be slipping.

I’m not quite sure.  I reassembled it with all fresh springs and that seemed to solve it.  I didn’t change the trigger until after I got it functioning.  I think it was most likely the trigger spring.