If the camera back is leaking a bit each frame could be a bit different depending on the ambient light, how quickly you advanced the film etc. Kinda like a pinhole camera. One thing, if you have film to burn, try going into a dark closet, advance the film 4 or 5 blank shots, then take the camera out in the sun, don't shoot or advance the film, go back into the dark, rewind the film and have it processed. If it is the light seals you should see it in the middle of the film. Not the very beginning or the end. Use a normal negative film, not Cinestill, just plain old inexpensive Fuji or Kodak.