Caused by not fully inserting the film holder into the back. On the film holder, next to the cutout for the frame, opposite the end you insert into the back, there is a ridge running across the film holder from edge to edge.

On the camera back, there is a groove that the raised section fits into when the film holder is fully inserted. If you stop when you feel the raised section first bumping the back, it will leave the film holder slightly outside the proper position and that last bit of film will not be exposed, even with the dark slide fully removed.

You need to push the film holder in, on past that first bit of resistance, until it "pops" into place with the film holder ridge seated in the camera groove..