boiler will not start up automatically when the room thermostat is asking f?

boiler is running fine producing hot water and heat, but when the room thermostat asks for heat the boiler will not respond. The blue light on the boiler is flashing. The only way to get heat is to manually re-set the boiler, then the boiler responds. My plumber has been over it and can find nothing mechanically wrong, is the control panel faulty
boiler is a worchester 24i installed aprrox 3 yrs ago