Is it under warranty or did you purchase it used? Tamron will still offer service on the RF645. I've never had a problem of any type with mine. I think maybe you just got a little unlucky with yours. I have both the RF645 and the Mamiya 7 and actually find myself using the Bronica much more, but they are both great cameras.
See the last few pages of your manual and try using the winder release (small hole on the backside of the top deck, basically below the tip of the winder lever) to see if that helps before sending it off to Tamron.
I had the same problem with mine. Got it fixed under warranty by Tamron. Got the camera back in a week via Robert White where I had just purchased the camera. Within a few months, the rangefinder patch got a bit out of line. I then lost my trust in the camera and sold it.