That's what happened to mine, so I overhauled it.

http://www.bikeforums.net/showthread...-rebuild-guide

Pics coming soon.

If you're not game to disassemble it, I'm pretty sure it was just a matter of gummed-up grease... but I'd already tried to fix it by squirting WD40 at it, so you might have to get the lever off the body and soak it in degreaser or something.