Maybe look again for mites? In the dark while they are roosting, with a flashlight, around the vents, under wings, and neck.
There are sprays, or try neosporin.
Mary
Try a food upgrade, for example, the Purina Flock Raiser, if it's fresh by mill date where you shop. My birds didn't do well on that Dumor feed, or layer feeds in general.
Paying more for a better basic feed is less expensive than adding ??? stuff!
Mary