single zone of muti-zon system not circulating

I have a boiler with 3 (taco) zones (air handlers on two floors and an indirect water heater) and a single circulator (taco). For some reason one of the zones provides no heat. I noticed when the thermostat calls for heat the air handler kicks on, the zone valve opens and the boiler fires up, however no circultion of hot water occurs. It was working fine, then intermittently started this behavior which is now constant. If the circultor works for the other zones what could be the problem?

air in one zone can prevent the circulator from moving water through it. The typical head of a circulator pump can't pump through air, but it can push water since it is aided by that falling down from the high side, but it may not be able to build enough pressure to push it up the hill otherwise.

Thanks. I thought about that but I can get it to serve heat if I shut the other zones off completely and recycle the thermostat setting calling for heat. But not always.