Exactly! Conventional chick raising calls for 95 degrees the first week, 90 the second, and so on. I had day old chicks under the heating pad cave and was asked what the temp was under there. I didn't know because I didn't really care - my chicks were thriving and that was all I needed to know. But out of curiosity I put a wireless thermometer under there. The room was 69 degrees - the cave was 82.5. So there went that theory!!
If the chicks don't want to be under it anymore, they may simply not need it. Or it may be too warm with their new feathers coming in and bigger bodies in the same space their tiny little bodies once occupied, adding to the heat. So you can turn it down a notch and watch them. You can also raise the frame a bit, especially helpful if they are having to almost have to crawl under rather than just duck.