Give ZooMed a call for sure, they should be able to help you troubleshoot the pads and see what's going on. You could just have the worst heat pad luck in the world, who knows!
Just checking the obvious, but you're plugging in just the heat pad to the wall, even the new ones, not using the thermostat initially yet, right? Just checking to make sure the thermostat isn't the culprit.
-Jen