My macbook Air is not connecting to a projector despite the right cables being attached. All suggestions welcomed!

My 11" two month old Macbook air is not connecting to my projector at work. I have the right thunderbolt cable extention, but there is no display. When I go to settings there's no projector detected, although when I remove and reconnect the thunderbolt cable the screen goes black for a second or two - so it is definitely connecting to the cable itself. Is there some obvious thing I'm forgetting to do?