RR has covered the likely issues here already.
At night, with a flashlight, look at vents and necks, at least, for lice and/ or mites. Check at least several birds, because they may not all have equal infestations.
Are the mill dates on your feed bags recent? The feed sounds fine, but old feed isn't good.
Do you have birds with nice feathering?
Remove that rooster, and be suspicious of any beautifully feathered hens, as possible feather pickers.
We have also had a couple of hens with poor feather quality, and removed them from our breeding groups. And an obnoxious rooster is not an asset!
Mary