Tub setup/heat tape PROBLEM!

I currently have (5) 15 qt. Sterilite tubs sitting on top of 3inch 6 watt/ft heat tape. This is sitting on a thick plastic utility rack, with shelving that is not ventilated or slotted in any way. I have this set up attached to a Herpstat ND.
My problem. The tubs are only reaching a max. temperature of 86 on the hot side and 79 on the cool side. I have another tub that has two hygro/thermos as well and that one is only at 77 degrees hot, 72 cool. I have bumped up my thermostat to (105) to compensate for the ranges they should be at inside the tubs (90 t0 92) hot and 82 to 85 ambient air/cool. I do have the thermostat probe directly on the heat tape.
MY PROBLEM is that the heat tape, at full unregulated capacity is still not getting the enclosures hot enough. What is the problem?
Also, the temp. in the room in which the tubs are, is usually 68 to 72...
Any help would be great. Thanks