First, have you treated your infested chickens with permethrin or spinosad, both effective and organic insecticides? Do that first, and then repeat it in about ten days to get any lice that have hatched from nits. If your rooster is weak, it may be from mites which might be a plague in the coop. It can make chickens anemic as the mites suck their blood at night. Both can occur at the same time, and you will need to spray down your coop to rid it of mites.
The nits glued to the feather shafts are like cement, and it's impossible to remove them. You have two choices. You can yank out each nit encrusted feather or you can ignore them, since all lice have been killed, until they are replaced naturally by next molt.
I have yanked nit feathers. I do it after soaking the chicken butt in warm Epsom salt water which makes the feather removal easier on both you and the chicken. Replacement feathers quickly grow in.