Each zone has it's own thermocouple (to measure the temp in that zone). Perhaps the thermocouples need replacing. In our coach these are round, somewhat raised devices hidden in areas that do not stand out, but do get the temps of that area. Ours seem to be under cabinets.
I suspect that the one that goes on is the cooler one and the others are at their setpoint. You can increase the setpoint at the thermostat for the zones in question to see if they come on. The Aqua Hot system will run even if only one zone, or the hot water system is calling for the boiler anti-freeze to be heated.
Remember this system is a closed system with a boiler anti-freeze (this is special anti-freeze. DO NOT use regular radiator anti-freeze) that is heated, then pumped through the system. The heat exchangers have fans that blow across the fins that is heated by this heated anti-freeze and send warm air into the coach. If the thermocouple tells the system the zone it is in is hot enough, the circulation pump for that zone and the heat exchanger fans do NOT turn on. Only those zones that are cooler than the thermostat setpoint will operate.
If the fuses for a particular zone were open, that zone would never come in, even if the zone were well below the setpoint temp.
FMCA #F431170, GS #822128658, Escapees SKP #112655
2012 Airstream Mercedes Interstate Class B