Still none the wiser why the feathers came out no sign of any new ones.they eat well and lay well
What do you feed for them? Have you given them any supplements?..any extra protein? When chickens are trying to grow new feathers, it is very challenging time for them and it will take lot of out of their bodies. If they struggle, it is good to supplement their diet with growers pellets/meal to give that little bit extra for hard times..it might slow down their laying for while, but it is better to allow them to get back to health than keep them running 'low' and fully laying.
When my girls are moulting I give them some poultry tonic in their water, I give them some complete dog biscuits (soaked soft in hot water) few times a week..not a lot, just small cup between 4 girls, thats their extra protein portion and little bit of poultry spice or seaweed meal with their pellets.
Do you find any feathers in their coop or pen...do they plug themselves? Have they even tried to grow any back?..any feather 'pins'? Have you dusted their house since the treatment?..louse powder or Diatomeceous Earth would be good for having around their housing all the time..I use DE all the time, all year round and there is no need for any other treatments...works against all blood suckers.