Well, it can take a while for those feathers to grow back. If it's mites or lice, not only do you need to treat the chickens, you have to clean out their entire coop and run, and then treat that too. I believe, with the poultry dust, you have to re-treat them again in 7-10 days, because the eggs then hatch, and you want to kill the young parasites before they reach reproductive age.
Another problem could be that there could be one really nasty hen who is pulling out the feathers of the others. Look for a hen that doesn't have feathers missing, or better yet, sit and watch them for a while, and you'll see if you have a feather plucker.
Diet can also be a factor.....are you feeding a good layer food to your hens? If they're not getting enough protein in their diet, they will literally pluck and eat feathers (feathers are almost all protein).