I have a problem with two sheet film holders. When I load the two sheet film holders, I am very careful to make sure that the film is in the grooves on each side and the dark slide is about half way in. Then I close the flap and push the dark slide all the way in. Yet, sometimes [way too often] after I have taken the photograph and put the dark slide in, the film has pulled away far enough from the film holder base that the dark slide pushes the film into the camera chamber. The result is a film jam and the negative is lost. What am I doing wrong and how do I prevent this problem.