There is your issue.

At an Ambient temp of 66F, this is on the chilly side for bullfrogs. Like all ectothermic animals, their metabolism slows down considerably with the temperature. The result is they eat less and become sluggish. Bullfrogs tend to be more active during warm humid weather, so a temperature around 75-78 F during the day and 70ish at night are probably best.

I will caution you on using off the shelf heating units designed for reptiles (which all of them are). These are a surefire way to kill your amphibians.

It is better to turn up the thermostat on your AC and keep the room warmer. It is also better for your electric bill...