You mentioned in your other post that this behavior started after several plants died in the tank. If you haven't replaced the plants they could just be hiding where they feel secure. Adding more plants (live or fake) will provide some cover and they might feel more comfortable coming out into the open. From a frog's perspective a wide-open area with no cover can put them at risk of being ambushed by a predator, so providing more cover actually results in seeing your frogs more.