I had a similar problem - the center terminal where the light bulb screw in became somewhat corroded. If you turn off the power, you can clean these with a small piece of fine sandpaper - then bend them slightly outward so that when the bulb is screwed in, they make good contact. If that doesn't work, and you have a voltmeter - check the voltage at each bulb socket to make sure the internal wiring is ok. Not to overstate the obvious - but make sure the bulbs are ok too....