Motorised valve not fully opening? Often the timer and thermostat open the valve which has a switch and it is the switch output that is wired to the boiler call to heat. When the synchronous motor gets old it doesn't open fully and so doesn't activate the switch.
Worth finding out if you have any motorised valves and where they are. They can get unreliable with age but changing the motor or head is easy if you are comfortable with electrics.
Motorised valve not fully opening? Often the timer and thermostat open the valve which has a switch and it is the switch output that is wired to the boiler call to heat. When the synchronous motor gets old it doesn't open fully and so doesn't activate the switch.