lost feathers and beefy red

My white leg horns and golden comets are just over 1 year old. The began what we thought was molting this spring, but it hasn't stopped. They have large beefy red backs and butts and some have thinned places on their necks and chests. I began to think it was mites or lice, but after weeks of poultry powders and sprays and multiple deep cleanings of their coop, they are still in this state. Their new pin feathers are not developing…seem to be turning dark at the root and not maturing into feathers. HELP!