hmm, try toggling the switch, something might not be making good contact. that is all I can think of. If not, how old is the lens, if is is under a year. Send it to canon with the receipt, and they should have it fix for you pretty quickly.
Super Frequent Flyer, no joke. Ex Patriot and loving it.
Canon Eos 60D, T1i/500D, Eos1, Eos 630, Olympus EPL-1, and a part time Pentax K-X shooter.