Substrate not warm enough?

[Craiga 01453]

I have a 10 gallon tank and a uth by zoo med made for 10-20 gallon tanks plugged into ipower thermostat set to 91 degrees F. I can feel that it is at 91 when I feel glass but the top of my substrate is only at 79f. It has been only 45 minutes should I be worried?

Sometimes they take a bit to warm up.

Unless you're using a temp gun, or at at least a probed thermometer those temps must be jyst a guess.
There's no way we can "feel" temperature with any degree of accuracy.

Also, it's important to focus on the actual surface temp and not the substrate temp. Snakes burrow, push substrate around, etc...and will end up laying directly on the surface.