Did you stick the pad directly on the bottom of the tank? When doing this, did you bend or otherwise crunch the pad, or remove it a few times to reposition it?

The ZooMed guidelines for their UTHs warn against this, because it can damage the wires inside and cause malfunctions and 'dead zones' on the pad.

Have you measured the ground temp on the hot side? If it is around 90-95, then your pad must be working. Do you have a thermostat to regulate the temps on the pad?