glowworm boiler wont light?

I my glow worm boiler will not light on demand i have to turn the power switch off and then back on again to get it to fire up, when it is alight it works fine but when it reaches the thermostat temperature and goes off it will not fire up again when the temperature drops so i have to go and switch the power back off and on again to get it fired up again, a puzzling task.