After a certain point, mother hens will no longer accept new chicks - particularly chicks of a different age than their own. It can be dangerous to put newly hatched chicks with older chicks, because brand new babies need to sleep under mom for most of the day and night, while older chicks just nap under mom occasionally during the day. Mother hens often become excited a few days into chick-raising and begin to kick bedding around; this is another danger to small chicks. I've seen chicks get tossed around by a hen who wants to forage.
A chick who has not pooped in more than a day may have pasty butt. See if there are droppings dried onto his rear end. You can clean pasty butt by softening the dry poop with a warm, wet paper towel. When it is soft, gently wipe it off. It's important to do this or else the digestive system can get backed up.