The most likely cause is a problem with the film ejection rollers or film path, not normal exposure or handling. Instax film develops when the camera’s rollers squeeze and spread the developer across the sheet as it ejects. A wavy undeveloped/white band usually means the developer isn’t being spread evenly.

Based on the symptoms, the rollers may be misaligned, dirty, partially stuck, or the film pack may not be seated correctly. Check that the film pack is installed properly, and inspect the rollers and film path for dirt, dried chemistry, or debris. The rollers should turn freely and look clean.

If cleaning and reseating the pack don’t help, the camera likely has a mechanical fault in the roller assembly. With repeated failures across multiple packs, the camera is the more likely culprit than the film.