Do I have crummy holders, or am I just bad at loading my 4x5 film? Sometimes my film "slips" and i get a 3-4times too big margin on one short end of the film and the image coming off the other edge of the film. I get the film underneath both rails but this seems to be some slippage that happens after it's loaded and maybe jostled around in my backpack. Do you guys tap your holders a certain way to try to shift the film back into place? I guess you'd need to tap the pulling out the slide end to get it to slide that way.