sounds like you have not treated their living area and they are just re-infecting. Doesnt matter what you put on them to kill the bugs, if you don't kill them in the nest boxes, perches, any wood surface, they will just keep coming back. They can live half a year with no chickens, then suddenly come back with a vengeance if you do not spray the entire area, weekly for a few weeks, and dust the hens at the same time. I had the same problem until I dusted ALL the hens EVERY week and sprayed their perches and all wooden boxes at the same time. It was a big chore but it was worth it. I have noticed some coming back again, so I will be back at it next week. I have to work this weekend so I couldn't get it done. Good luck. Hope you get it licked.