How old are they and what setting is the pad on? Also what is the general temperature of the space they are in? Even at a few days old they would ordinarily spend a fair amount of time out of the cave if the pad is warm and as
@bobbi-j said, they can get their backs up against it. It really does work much like a mother hen. The chicks come out and eat, drink, play and go back under when they start to get cold. When they warm back up they come out again. The big difference is that the hen can follow them around so they can explore their environment and still be near their heat source.