You most likely have worms. Even though you can't visibly see them, the eggs can be seen under a microscope. You should get a float test from a vet and begin deworming the whole flock. Clean out all the droppings in the coop that you can get so it won't spread after deworming.
We recently purchased a beautiful Andalusian in October from McMurray's Hatchery from a group of six pullets. As it grew we started to notice how long feathers were growing from its neck, a large waddle and comb, and increased skittishness. We already (accidentally) got a rooster from purchasing...