New Flightcom 403MC Troubleshooting

I have a weird problem that I have never seen before with brand new 403MC installation. I wired up the radio and intercom exactly like their installation sheet shows. 4 place intercom. I used my Zulu ANR headsets for the setup. Everything appeared to work great. Intercom, side tone, tx/rx etc.

However, if I use a standard David Clark or Sigtronics headset I get nothing. No intercom or side tone. But if I plug in a single Zulu into any of the positions, it all starts working fine again.

Any ideas why my Zulu's would work fine and but not a dumb headset?

I called Flightcom this morning and the guy was stumped. He is supposed to run it past his team and get back to me.

I vaguely recall that the DC?s use a mil spec plug, and are sensitive to jack tolerances.
1. Look at the DC plug. Make sure the plastic handle does not keep the plug from being fully inserted.
2. Try pulling the plug out of the jack, just a few mm. If that works, put a washer on the plug to keep from inserting it too far.
3. Brass tarnishes. Run some steel wool over the plug.

My guess is #2.

I thought about those simple things too. It doesn?t work with my Sierra headsets either which have an identical plug as the Sierra.

stereo vs mono headsets? Do you have the TRS jacks wired correctly?