Is it one of these? If so, there's not much to go wrong with them so it's probably just gummed up with sh*te. I would just dismantle it, clean it and bung it back on. 5 minute job...
Good thing but i find mine awkward to set, feel it shoud turn the opposite way to tighten, i know there are safety issues with that but theres also safety issues with setting the damn thing. IMO at least.