The best place for the probe is between the UTH and the glass. You didn't cover the probe itself when you taped it on did you?

You need to remove most of your substrate, UTH's do not affect the air temperature in the cage so all the heat that they can provide is what comes up through the substrate. I have found that the substrate layer needs to be no thicker that 1/2 Inch.

What type of thermometer are you using?