It happens that DerbyBorn formulated :
The call for the pump and boiler comes from the switches in the valve as
far as I know.

Correct!

Timeclock feeds to stat, stat feeds through to valve actuator, when
valve actuator has set itself to the correct position, its micro
switches then feeds through to the boiler and pump. On more modern
systems, it just feeds through to the boiler alone and the boiler then
turns the pump on, that allows the boiler to run the pump beyond the
time the boiler is firing, to remove heat from the boilers heat
exchanger.

So if the valve actuator fails to achieve it required position, the
boiler will not fire. Sticking actuators, especially the spring return
type are a common fault.