Thanks. Seems like operator error. The non-C, NB and C lenses have a selector knob for MLU. It’s possible to put the knob in MLU position, fire the body, forget to fire the shutter, then advance the film and recock the body. I have done this, more than once on the same roll. It is easy to think you have exposed the film when the body is fired. The film back only knows when the body has fired. There is no communication between the lens shutter and film back.
If this doesn’t sound like your situation then there could be a problem with the lens. You can test the film back + body + lens without film. If the testing shows correct function, then the original problem was operator error. If the lens has a problem, time for a new lens-probably cheaper and more available then a CLA.