I have a Mamiya-6. A couple of days ago it was working fine. Today I put in a new roll of film, closed the back cover and started to advance the film with the film advance lever. But noticed two strange things:  The shutter button would not function, I would press it, but nothing happens  The film never stopped. I could advance the film with the film-advance lever endlessly (without having to press the shutter button, which as mentioned, would not function). So I removed the roll of film to try and figure out what is wrong. Strangely, When the back cover is open - the shutter button WORKS! It clicks! But then when closing the back cover again - it stops working. I press it and no click. And I can pull on the film-advance lever over and over. Any ideas what might be wrong?