When the film bulges in the film gate the focus will shift behind the subject.
You can confirm if the is the problem by taking the lens off and exposing at "B" with film in the film gate. When you touch the tip of a pencil to the film it should not move if it is against the pressure plate. If you see the film move or indent when touched with the tip of the pencil, the film is bulging.
The cure for this is to shoot 4x5 film That is what I do when I don't want to be bothered by rollfilm flatness.