I have a 135W back for my ETRS - and it's one of the trickiest pieces of equipment I've ever come across. It is possible to do everything wrong, with results like you describe. My best advice it to sacrifice a roll of film (35mm) and a couple of hours. Sit down with the bach and film, try every possible way to put the film in the insert (the correct way is not obvious), try to advance it, check that the emulsion is facing out (!) etc.

Good luck - it's been so long since I used mine that I may have to do this all over again...