Are you certain it has a high impedance setup? If you try a low impedance headset (such as military stuff) in a civilian setup, the mic doesn't work... I seem to remember my low imp one working on one or two civ helicopters but not all of em.
Could be a bad part too, but if it works in some but not all, that would be my best guess. I believe they make adapters out there, tho it might set you back a few bones.
-Mike