Might want to do some research if you haven't already done so to first ID your tree frogs and then learn about their habits. Are these wild/local tree frogs as opposed to those sold commercially? My knee-jerk response would be that perhaps -- if their wild locals -- they are getting into hibernation mode. At least here in MI Fall is fast approaching and we've been having quite a few chilly nights. You might find some answers at the forums on kingsnake.com

: )

Here is a link that might be useful: kingsnake

I have recently caught a local wild green tree frog. And I have also noticed that fall is coming. But I caught mine on a 55 degree night so mabey some species hibernate but some don't

I know that my two White's tree frogs sometimes like to nestle into the dirt/substrate, though because there's gravel in my tank they prefer a small rock tunnel. Maybe there's too much light, and their just hiding out until "night" (lights off). Check awhile after you've turned the lights off...do they come out then?

Tree frogs need a good deal of humidity, it is very common for them to bury themselves if the tank becomes to dry. You may have a bowl of water or another wet area but if the air is dry they will seek out the moist spots.