If the mechanism works OK without film, there might be a problem when film is installed due to the very slight increase in thickness. The back area where the little frame counting tang is depressed when the back is closed may have become worn just sufficiently to allow a problem (???)

Did I not read where you ran a whole roll of film through accidentally? Why not use this as you “dummy” roll and do some experimenting. Actually b&w roll film is so cheap, I would advise you purchase one (or two) just for learning. You could also use the film to learn tank loading if you are thinking of home processing.