I am using a taco valve model 555-050RP. I have 4 zones to my boiler. The 3rd zone does not activate the boiler. When I checked the voltage across 1 and 2, I get 23.3 volts when the thermostat is turned up of course. I have jumpered the thermostat but I am not getting 25 volts to the taco valve. I have already replaced the taco valve before checking the voltage, because I assumed this was the problem. I was thinking I may have a wiring issue, but my wires going from the 3rd to the 4th valve are working properly because the 4th zone works fine. What should I check next to find out why the 3rd zone is not getting enough power and how do I fix it?