First of all, how big are the tanks and what size UTH's are you using? The UTH needs to cover at least 1/3 of the bottom of the tank, but not more than 1/2. What are the temps on the top of the substrate on the warm side (not the air temp)? The temps on the warm side need to be between 88-93 degrees, and best kept near those temps 24/7. And yes, if the temps are too low your leos can stop eating. They need belly heat in order to properly digest their food. If they can't get sufficient heat they often times will stop eating.

If you are not measuring your warm side temps, you need to get a digital thermometer with a probe to do so. You can find these pretty cheap at Walmart, or check with your pet store. If your UTH's will not achieve the proper temps you can add overhead lighting for additional heating. The infrared (red) bulbs are the best for this purpose as they can be left on 24/7. Infrared bulbs have very little visiable light and will not interfere with a leos day/night cycle.

Generally you don't have to worry that much about the temps on the cool side of the tank. As long as your warm side is where it needs to be it seems the cool side falls within the 75-80 range which is recommended. It may be cooler at night, that is fine.
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