<p>Hi,<br>
at first you could try to remove the back, cock the shutter, point the camera to a light source and release the shutter to see if both, the secondary shutter and the lens shutter are working correctly.<br>
But to me it seems more like the back has sticky mechanics, e.g. the back doesn't stop the film winding at the right point? Have you got a second film back to check?<br>
Regards, Wolfgang</p>