Many new users of Mamiya RBs tend to fire the shutter, and as soon as they hear the mirror go up they move. I see this all the time with my first time MF students. It doesn't work as fast as 35mms.
What you don't realize is the shutter fires after the mirror goes up. This lag time is what causes your out of sharp pix. Hold on one more second after you hear the mirror and your shots will start to clear up imediately.