What thermostat? How much wattage total is the heat tape, what size is the tape, and is it one continuous strip? Most racks come with a grooved slot to slide the tape into and may mean the probe is not flush against the heat tape but leaving a small air gap. It should be in physical contact with the heat tape perpendicular to the black lines. So how do you exactly have the probe installed and seated against the heat tape and how exactly and with what device are you reading the temps? What is the surface temp of the tape?

All that being said hte most importatn thing to ask is what are the room temps fomr the bottom of the rack to the top? If the room is varying to greatly form bottom to top it might be impacting the location where you are taking temps. Things such as ceiling fans, HVAC vents, radiant heaters, etc.