Most of the chicks and guineas I have lost in the brooder have been light as a feather when I picked them up to dispose of them, like they had dehydrated. There was water and food and plenty of room to get away from the heat source. I even put a screen across the top of one end, so there would be better air circulation.
Yesterday, I started putting the new hatches from the incubator beneath the hens that have new chicks. I will be curious to see if the survival rate is better with a hen, who will teach the little darlings to eat and drink and help keep them the right temperature.