Gas fired hot water baseboard system. 3 thermostat zones. When 1 of the zones calls for heat, the zone valve opens and a 'click' is heard, but does not kick the pump on. If I switch the pump from 'intermitent' to 'constant', water flows, the boiler kicks on and all works fine.

Not sure but if it's a relay or a contactor that connected to the pump, that could be the problem