I am having a similar problem with one of my zones. Blowers aren't coming on in either bath register. I checked the fuses so after talking to an aqua hot tech he narrowed it down to the following:
1. If the pipes to the register are hot it could be a bad pump.
2. A bad relay
3. A problem in the motherboard and harnesses.
4. A bad fan BUT he has not had to replace a fan in his 20 years of servicing.
Just thought I would share that in hopes of narrowing it down for you. I'm taking mine in for service/fix next week to an aqua hot service center.