My problem is already solved, but I thought I would post about it to help others who may experience the same issue.
I recently noticed that my THC was not taking any action to correct the torch height, even though the voltage was being reported correctly and there should have been some THC control. I started watching the THC lights and the Z axis height for any sign that the THC was working. There was no lights on the THC control and the Z axis didn’t ever say “under THC control”, even though it should have been. The H1 and H0 commands were in the code, so that wasn’t the issue.
I also noticed that the Feedrate shown in Firecontrol was faster than my programmed feedrate. I went back to Sheetcam and checked the feedrate on my tool and it was correct (100 IPM). The .tap file still showed a faster feedrate for the program speed on the last line (PS120)
I checked my path rules and remembered that I had recently set a path rule to do the lead in at 120% of the program speed, to see if that helped with defects at the lead in location. The post processor sets the program speed comment at the end of the code based on the fastest speed in the program, not the programmed cut speed.
In this case the fastest speed was 120IPM for the lead in, so the feedrate was shown as 120IPM in Firecontrol. With the program speed actually being 100 IPM, that meant the torch speed never reached 85% of the feedrate shown in Firecontrol. This means that the THC would never turn on, because the torch speed is always below the THC cutoff speed.
I removed the path rule to speed up the lead in and the problem was solved. The THC is working correctly again.
I use Sheetcam, but I suspect that Feedrate Optimization in Fusion would cause the same issue if you use it to speed up faster than the normal cut speed.