I just replaced a dead Taco power head the other day. The 'clicking' that you hear is a mechanical switch inside the valve. It is what sends the signal to the boiler that heat is being demanded. When your thermostat is requesting heat, it may take up to a minute for the zone valve to actually open. If you have hot pipes on both sides of the valve, it would sound like it is working, but if the signal from the valve may not be getting to the boiler, your circulator pump won't know to turn on.
Easiest to troubleshoot with a meter. Measure the current from the thermostat to the valve. You should have ~ 1 amp when the thermostat is demanding heat. Once a steady-state is reached, the current will change between zero and ~1 amp every few seconds. This is the servo-action of the valve. If that is working ok, then check that the signal from the power head is getting to the boiler that tells it heat is being demanded.
If you have a lever on the power head, flip it up towards the pipes. This will manually open the valve and manually activate the switch. At this point, your circulator pump should be running and hot water flowing.