Ive had them where the small brass part for power is sunk in the plastic or worn down as to not contact the bulb part.I usually pull them out of the plastic a bit and try to make them stick out further.Ive also had some where the socket isnt touching the bulb brass for ground.Try slipping a real thin screwdriver thats grounded between the bulb brass and the inside of the socket and see if it lights.Rocky