My rack i have now, the first one i have built, i have 4 shelves with four flex watt heat strips in parallel as well and i encountered the same problem. From now on i think i am going to just run one continuous strip and snake it through the rack. This way so i know all the heat tape is functioning and it should be all at the same temp.*

*Note: I would expect maybe some cold spots in the heat tape, but this argument really boils down to the quality of heat tape.

Just wanted to add my two cents, and what i have learned. But i would definitely agree with the other two have previously posted.