Heat mat isn't hot enough.

[Marzipan]

A mat is only for a hot spot so I hope you are not trying to raise the ambient temperature.
Your thermostat probe shoupd be between the glass an mat.
You should be taking your temperarure under the substrate.

Thanks for the fast reply!

The ground temp is reading at about 24C. The thermostat probe is right on the bottom of the RUB (no substrate or towels in atm, just doing a test run before S-Day). This is on the hot side, right above the heat mat.