You have to understand that when you cram so much film onto a reel and each layer is only a few mm distant from the next, the flow is compromised. The sprocket hole areas allow a free flow of developer and with the areas for which there are no sprocket holes, the flow is somewhat restricted. Solution?

I agitate continuously, gently, and leave no area of the film untouched. It's somewhat like washing clothes on the gentle cycle. I never have that problem. It might be a bit of a hassle to keep moving all the time but isn't it easier that dealing with the aftermath? Bromide drag can leave streaks down the film, in the direction of gravity. - David Lyga