So I bought a T8 AP cage, set it all up and I have a 11x17 ultratherm heat mat on a thermostat under it.

but I have the thermostat set to 100 degrees but inside I can't get it over 83. On the hot side? I haven't had a chance to measure the cool side yet but I'm assuming it's somewhere in the 70s. How can I get the heat up? Do I need to buy another heat source? The mat is flush against the bottom right now and the probe is reaching up to 1"" as its set to but my inside thermometers aren't getting over 84 ever. They're just going back and fourth between 83-84