I still very much consider myself new to LF, I'm trying to learn to use a 4x5 (Cambo). I recently shot a number of sheets while on vacation and just got around to starting the developing. It's TMax 100, D76 using a CombiPlan and Fidelity Elite holders.

Most of the sheets seem fine, but more than a few have a very broad, unexposed band which ends up making the processed format almost square (see attached image).
When I load film, the flap end is away from me with the film notches in the right-hand corner. I withdraw the dark slide enough to reveal the end of the film slot/track. Then I insert the sheet and make sure it is snug against the stops at the flap end before closing the flap and seating the dark slide.

It almost looks as though the sheet is riding up in the holder towards the open end. Could pulling the slide when shooting allow the sheet to ride up and thus a small strip never gets exposed? It seems unlikely. I've just tried a couple of the holders, one of which I'm sure was an offender, and there does not seem to be any way a sheet could ride up over the stops at the slide opening end. Any thoughts, suggestions would be appreciated. Thanks.