You don't describe your technique (roller processing, inversion agitation, twirling agitation, how much developer you use, etc.) so we are just shooting in the dark, but:(posted 8322 days ago)If you using inversion or twirling agitation, you must put a blank reel above the full one to keep the full one in place.
Use only as much developer as is needed to cover the lower reel, assuming you're not at extremely weak dilution. This allows empty space for the development by-products to equally distribute when inverting.
With normal developers, using a two-reel tank, top reel empty, and only covering the lower reel with developer is my STANDARD method. I've always gotten consistent development.
By the way, use empty reels and add water to cover the lower reel. Measure how much water was needed to cover the lower reel, so you can go my measurement, not estimation, on how much developer is needed.