I have one chick out of an incubator-hatched batch of nine that isn't growing. Its siblings are thriving. They are four to five weeks old and with a foster mum.; they were put with her a couple of days after hatching. She's one of our ever-reliable Coucou de France hens so hasn't slacked in looking after the chicks. This one has grown its wing and tail feathers - and then just stopped. Currently it's in a breeder with two twelve-days old chicks who are about the same size (one is a Sussex, the other a Maran + Coucou cross); it's feeding and drinking and foraging around happily. Any ideas why is isn't growing?