This has happened to a couple of my C330 lenses, too. Just the lubrication getting old and sticky (if it isn't the flash lever in the wrong place - as Erik suggests)... a CLA will easily restore them to full working order...BUT!

The C330 is officially obsolete. I took my lenses to a Mamiya dealer recently and they wouldn't touch them. They recommended someone else but they had gone out of business. An APUG member suggested someone to me... but they never answered my email. I live in the UK - hopefully things may be better where you are! It really shouldn't be a difficult job for a competent repairer of any mechanical camera shutter.

It looks like I might end up doing the CLA myself. I have some engineering ability - and I've tackled many folding camera shutters before.... but If anyone can suggest a repairer in the uk - please let us know, I would still prefer a 'pro' repair, as this is my *best* camera outfit.