I would need to see the prints in question to make a reasonable response. But as a shot in the dark, have you checked for light leaks in your darkroom, including your safelight? I once had the dye flake off a red safelight bulb resulting in a pinpoint of light. This evidently produced a rather narrow beam which, from time to time, fell across my paper resulting in bands of slightly fogged areas, which ultimately resulted in print banding. It took some time to figure this out as I never handled the paper in the same manner or located the box of unexposed in the same place.