Are you running the tape down the back? How wide is the tape? What size tubs (including dimensions) are you using. Are your tubs in the rack sideways or front to back? Are you using a thermostat or dimmer to control the temps on the tape?
You say the tape is barely heating the back, have you taken a temperature reading with a thermometer? What type of readings are you getting?
When you say you think the sides are to short, do you mean that the sides are not completely covered front to back? The shelves on my rack stick out past the sides. The tubs slide in length wise. I ran 11" heat tape under the bottom of each tub and use a thermostat to control the temps. My rack is in my basement which tends to be around 65 degrees at night and 70 degrees during the day. I have been able to consistently heat my tubs.
If you answer some of the above questions, someone may be able to point you in the right direction.
Nick