Keeping film in place

Hi. When scanning film, how do you lock the film in place in the so the nessesary amount of tension can be made? For me it is not enough just locking the film as you would do normally when using a projector.

Not sure I understand the question. The film maybe in place by equal tension on either side of the gate. That tension is defined by friction against components along the path and by holding tension of the pickup and supply spools.
Can you elaborate on your film path to understand why the film is sliding?