If the "heater [is] not blowing", then the fan ain't turning or it's blocked.
The first I'd check is the fan. Years ago, we had one fan not blowing. A large nylon tie wrap was in it holding it firmly from turning. Somewhere along the line, a technician had cut a tie and left it under the dash. It eventually moved to the fan opening, slid into the fan blades, and stopped the fan from turning.
My experience indicates to me the fans are actuated by their respective zone thermostats. We can turn on the fans with no heat in the Aquahot lines. Thus, it appears the fans are not actuated by the Aquahot. So, the problem has to be the fan motor itself, or in the wiring between the respective thermostat and fan that is not blowing, or in the respective thermostat. If you can get to the leads connected to the fan, then perhaps you can use a multi-tester to check for power there and/or connect from a battery or power-pack to see if the fan will spin. While in there, check the leads' connection to the fan motor.
The next thing to check is at the respective thermostat. And good luck with that...!