Multi-roll small tank consistency problem

I use a Paterson two-roll tank to develop my B&W film in TMAX and HC-110 Dil B developers. Using both developers I am experiencing a difference in negative density between the top and bottom rolls in the tank. When I develop two rolls in the tank, the top roll results in significantly more dense negatives than the bottom roll, which looks quite right. I use 600ml of developer and am quite certain the solution completely covers both rolls. Any suggestions as to why this happens or what I can do to correct it?

Thanks, Robert

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