Good luck with your hens. Was just browsing here while waiting (hoping) someone will answer my question. I wanted to mention though - I used "garden and poultry dust" on my chickens last year for mites. It seemed to kill the mites.... but it also killed my favorite chicken.
She was a 4 yr old RSL - a hen with personality and intelligence I have never seen equaled. Long story short - she was hit by a car when she was 1, and I nursed her back. Overall, she was pretty much back to normal, but at the end of every winter she would sit in the coop a lot - I think her legs hurt her. Almost like she was broody except she wasn't broody (although the sweetie did hatch chicks for me 2 years ago).
Anyway, since she was in the coop a lot – I think that's why the mites got bad on her. So I dusted her and the others. Everyone else went outside and ran around, dust bathing and free ranging, etc., but my sweetie stayed inside. I would carry her outside every day when she did this - at least for a little while, and kept her separate from the others (partly to avoid the attentions of the rooster - which is why I think she really stayed inside). A day or two after dusting her, she stood in the grass preening herself, and I noticed there was still quite a bit of white powder on her. And here she was sticking her head in her powdered feathers while peeening.
I know, that wasn't so short. But I'll end it here because it's been almost a year and I'm still so upset with myself...
My baby went blind within a few days. I spent so much time trying to take care of her, but she was barely eating, and getting worse, and I didn't want her to sufffer any more than what I had already unknowingly put her through, so my vet put her down for me.
Anyway, I hope the product you used works and doesn't hurt your birds. I did have the same issue (feather loss) with one of my hens - I actually tried a hen saddle for a couple of months and it didn't help at all. It was mites, and once I got rid of them, her feathers regrew. Hope same for your girls. Best wishes.
~Sharon