Most common is too cold or too hot in the brooder. Should be 95 degrees in the hottest place the first week and decrease by 5 degrees each week, with plenty of room to get out of the heat. They regulate by moving in and out of the warm spot as needed. If you are using a heat lamp, check the temp with a thermometer. If using a heat plate, then putting it at an angle lets them be closer or farther from it as needed.