Sometimes that happens. Do you clean the rollers occasionally? It's a good idea at least to check them between packs. Check the instructions at the Polaroid site. Most Polaroid backs work the same way in this respect. The only differences are the mounting plate, and in the case of the 35mm NPC and Forscher backs, the fiber optic block. I believe the Hassy Polaroid back also has a glass plate to improve focus, but that's not relevant to the problem you're having.