Ideal W Standard Boiler?

The pilot light is on, but boiler is not firing up. If I press
the knob (for starting it) and keep it held in the boiler fires up, but as soon as I release the knob the boiler goes out. There is no problem with the pilot light. The boiler controls have been set to the on position. When pressed in there is an electrical clicking noise sometimes. Can anyone suggest a cure! Thanks