About 8 weeks ago my Sussex hatched 8 assorted eggs (2 Olive Eggers, 5 Araucanas & 1 unknown). She left the nest after the 7th hatched and I could hear a chick chirping in the 8th cracked, but not hatched egg, so I broke that egg open. All chicks are doing fine, except the one I assisted. The chick is eating, drinking & socialising with the others, but it is not growing feathers. I have inspected the flock & mites is not a problem. Has anyone experienced this?