Quote Originally Posted by Somed00d View Post
The way you have it is fine. I would plug that extension cord directly into the wall and see if the heat tape warms up, don't let it get too hot and melt keep an eye on it.. it should heat up quick. Try with a different extension cord. I have had a similar problem with a cheap extension cord causing one rack to not get power. Luckily it wasn't the one with the probe or it would have caused a disaster.
Right on
Wiring it in parallel would have been an easier solution for a rack allowing you to use one plug.
Here is the wiring diagram
http://www.beanfarm.com/heating/Flexwatt_parallel.pdf
Another thing is what size of Flexwatt are you using and what is the rating of the thermostat? Depending on size flexwatt is 8-20 watts per foot, so you need to make sure you are exceeding the wattage of the thermostat.