As air gets trapped in your boiler radiator, you will need to bleed the radiator valve to release this air and allow your radiator to resume heating at full capability.
Bleeding the Valve
Begin the bleeding process by turning off the boiler's electrical power. Locate the bleeder valve, insert the radiator key into the valve, and turn it counterclockwise until you hear the hiss of escaping air. When the excess air has finished escaping, you'll see water begin to leak out of the valve. When this happens, close the valve by turning it clockwise. Finally, turn the boiler power on again.