I’m using sheetcam, could my post processor have something to do with it?

If its not moving after pierce, check you are getting ARC OK  signal into mach from Proma and also that Proma is seeing the Arc voltage as being more than pierce voltage - there is a setting in proma for this. If its not sending arc ok that will cause the torch to freeze.

When testing, Mach is not going into reset or freezing is it - this can be a problem with noise from the plasma, if not, can you still jog it around AFTER trying a cut - is it still talking to the BOB/machine?

Just trying to rule out various possibilities for failure

Yes I can after I hit stop on my gcode.

ok made a test on my mill (cs-lab Motion Controller)

what i did:
1. loaded Plasma Screen
2. configured three Inputs Torch ON - Torch UP - Torch DWN
3. Turnen THC ON (Programm run Screen), also set Config -> Ports&pins -> Mill Option -> Allow THC up/down
4. Set THC Min and THC Max to -20 and +20 (mm i am working in metric, therefor i had to modify Screen dro's to see the sign)
5. then i was able to drive Z axis up and down within THC Max / Min range

hope it helps.
Thomas

BTW are you running Mach3 in Demo? remember that THC is not working in DEMO, but can't find this anymore.