Is the soil pretty dry? You want to keep it somewhat moist. Your heat pad will dry the soil out pretty quickly too.

Cover some of the top, but not all since your tree frogs should have some air circulation. A layer of sphagnum moss under the soil (you can separate with window screen material) will hold a lot of moisture also.

What are your room temperatures at night and during the day? Sometimes using a light bulb will be better than using the UTH pad since the UTH is heating the soil, drying it out, and then getting some heat to the tank where your tree frogs are.