Do you have a controller for the zone valves? If so, it may have what is called an 'end switch' that closes when any one of the zone valves is calling for heat. This gets wired to the boiler to request heat. Also, depending on the boiler controls, it might not fire if it is already within the min/max setting of the aquastat, so that could be normal.
Did this ever work 'right', or is it a new install/configuration? There are lots of ways to setup and configure a boiler, so it is hard to be exact without more info on exactly what you have.