Many zone valves have what is called an end switch in them that is closed when they are activated. This switch is often used to actually trigger the boiler to operate. That switch may have failed. It is pretty easy if you have a meter and can read and understand the wiring diagram. The zone valve motor may not be opening the valve fully, and that may be what is preventing it from closing the switch as well. I do not know if you can repair by replacing the defective part, or you may need to replace the whole thing (once you've isolated it to that as it could be some other failure), but I'd look there first. If the signal is getting through the end switch, depending on what you have for a controller, that could be bad. Again, it should be fairly easy to trace the 24vac control signal and find out where it stops. Since the thermostat appears to be activating the zone valve, it's probably not it.