First, the probe needs to be outside the tub, usually it's sandwiched between the tape and the back wall of the rack. If you put it in the tub the snake can move it. It needs to be placed where nothing interferes with it.
Second, heat rises so it's normal that your top tubs will be warmer than those on the bottom, especially with back heat. It sounds like your rack is giving you the same behavior no matter where you put the probe. One of mine is the same way, I just run BRB's in the bottom few tubs and BP's above them. I've found that belly/bottom heat is more consistent than back heat in a tall rack.
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