I have a Trane 5 ton 14 SEER heat pump with a Trane Propane backup heat source and an 3 zone Honeywell HZ322 zone control. One zone just developed an attitude whereby it gets heat normally when thermostat asks for it as long as he heat pump provides it. When the temps dip to force the propane furnace on, that one zone sits idle. The thermostat acts normally and switching with another upstairs zone didn't help a bit. I am a former engineer and now 12 year home inspector and see all sorts of stuff but this one baffles me. All connections look OK at the zone control. Any ideas? Thanks in advance.