Is the film brand you are using a little smaller that normal? The film just slides in, there isn't enough room to really cause these symptoms unless it is outside the rail. Have you tried loading a developed negative with the border problems in the holders, and check on how much wiggle room there is? Without putting the dark slide in, jiggle the holder and observe the film movement.
Should not be much. If the image is coming off the edge, then it cannot be under the guide.