I am no expert, but I suppose you have issues with the back, as there is no way, the body can "know" which of the twelve frames you just have exposed as you advance the film.
Yes, might be a problem with the back. No fault from the side of the camera.
First of all make sure that the film sits really tight on the insert and that you do not use too much speed when advancing the film after each shot.
If this doesn´t solve it then go for a CLA. How does the spacing look like? Does frame space increase over the film?
Film spacing starts out at about 1/8" and reduces to 0" on the 12th exposure. Trying to get another back from H. Chamberlain to compare. And I will get a CLA on this back once I have another to keep shooting with. The negs are super sharp and crisp. No problemo there and otherwise works fine and I can print/scan them as they are, it would just be better to have them spaced as we are used to seeing them. Thanks for the advice.
Logan
I have the same problem with one of my backs, never bothered about it , but at least I get 13 pic's out of a rol.....
As long as the frames don't overlap.
By the way: it is called frame pacing instead of negative spacing !