the contacts are probably not touching / run another white earth wire from n/side to off/side sometimes they get slightly corroded internaly due to copper oxide build up caused by damp hope this sorts the problem for you
george, February 2007
Upping the bulb wattage may not be legal. So you may want to stick to cleaning all the connections. The sockets tend to get particularly corroded so remake all the connections there.
Terry, October 2005
Clean the contacts for the ground or up the bulb wattage.
