Hello;
Look at the film insert carefully! Does the film pressure plate have enough tension? Beneath this is a flat spring with the edges turned up to apply pressure. Is the film insert correctly locking into the back? There is one lock on each side that must click into position. Also check for bits of backing paper that could be blocking the insert. You could also get a 220 back and remove the plastic interlock wedge on the side, which turns this into a 120 back. The backing plate has two grooves cut in allowing the plate too set closer in on the film rails thus applying more pressure on the backing paper. The film when loaded should be tight, no flopping. Hope this helps, Steven.