I agree with Ebony, sometimes frogs are just so stressed when you get them they seem okay at first and then they hide all of a sudden. Make sure you check your temp and humidity and try to get one of those digital gauges. The cheap dial ones sometimes don't give accurate readings. I have a ton of live plants in my tanks so that they can stay completely hidden if they want to. In fact I don't usually see my frogs at all until they wake up and start moving around. I keep them in an office that is not a heavy traffic area and is quiet at all times. You mentioned in your other post about a color issue on your frog. If you can post a picture, some of the guys here can probably tell you if its something to worry about. A lot of times they tend to change color due to temps, stress, humidity, night, day, whatever reason strikes their fancy. Most of the time its nothing to worry about, but like I said post a picture so we can help you out. Also if he's in a large tank try the cricket bowl trick that we've posted about. I use it to make sure everyones eating and how much they've eaten. There's a post on it you can read about here on the forum. Good luck and I hope he gets better.