Be sure to sprinkle the mite powder all over the floor and in corners, and in roost/nests.
THEN, my suggestion is to BATHE them, each!!! I had NO idea that chickens LOVE to be bathed, AND hairblown dried!!! I found this out by ACCIDENT (notice my profile pic is from 2 of our chickens winning at the National Western Stock Show 

 I didn't know until 2 nights before the competition that WE HAD TO GIVE THEM BATHS BEFORE THE SHOW??? Supposed to do so 4 days before the show, but we were not only new to backyard chickens but also never had shown before ... Anyway, so I spent hours googling youtube videos for how to "bathe a chicken" go figure, there really aren't ANY videos--namely how to bathe LARGE chickens (you CAN find videos on how to bathe bantams)... anyway, we also had to bathe our feisty rooster. DEAR LORDT Okay, so, I'll share what I found out, so that you can also try this--I highly recommend it--I think EVERYONE should bathe their chickens at least once every couple of months--for us, this has proven to be helpful in not only (hopefully) avoiding mites (so far) but also it allows us to REALLY inspect our hens (we had to rehome the rooster).
I bought 3 50-gallon clear bins (we always need them anyway), and they fit into our bath (they may not fit into a standard bath, so you could also either bathe outside during warmer weather, or place them in the bathroom in case there's some spillage...) Anyway, first bath has 1+ cups of white vinegar and 5 gallons or so of warm (NOT hot, and NOT cold) water... I place the chicken into the "soak" bath first. The vinegar helps kill any bugs, and loosen's up deep dirt, as well as helps to whiten any white feathers. I use a red plastic beer cup to pour the vinegar water ALL over their back, and up their neck (but, of course, avoid their earlobes, eyes, and mouth). I do this soak for 5 minutes at least...
Then, 2nd tub I just have WARM water, and I squirt Dr. Teals lavender bath soap into another cup--mix it well with warm water and pour it over their back, neck. I also put into this tub, a sponge to wipe their wings away from their body, and a baby brush, to scrub their shanks. I also make sure to lift their wings and really scrub--gently--under there with the soap, AND, I literally clean their vents with my bare fingers--until they are ALL clean and debris free.
Third tub is just warm water, and with another cup, I use to pour over them to rinse them off all the way.
I failed to mention first, all of the things to have on hand BEFORE you put a chicken into the fist tub... I have on hand: 2 large towels, which I wrap them into one, when I pull them out of the 3rd bath. I then sit on the floor, where I've put the hair dryer, AND, I put coconut oil (the solid variety) onto their shanks and I massage into ALL of their "fleshy" parts--the coconut oil is really good for their shanks and fleshy parts, but not on their feathers... Do you know that they have a preening gland on the mid, center of their back that they will start "pecking" at shortly after they're done with their spa... that is why you're supposed to give them 4 days after washing off their natural oils, to preen oil back onto their feathers... So, I mix in a tablespoon or so of the DE (diatomaceous earth) powder into the coconut oil--and I LATHER it onto their shanks, bottom of their feet, and in between their toes AND I rub it very liberally onto their vent. This is only impossible to imagine doing the first time 

 Putting the DE into the coconut oil and rubbing onto their shanks, definitely gets rid of scaly leg mites--so, again, there's NO harm in doing this even if you don't see anything...
THEN, I blow dry their entire body, as much as possible--I've had to be aware of how close I hold the hot air end to their body--like, I hold it a good foot+ away, and I have the heat set to low, but NOT cold. I also alternate to medium, then back to low heat, and I keep it at the low blow setting--but, you won't believe how much they LOVE getting bathed. While I'm blowing dry, I am inspecting their skin very closely, and the base of all the feathers, where the other mites tend to burrow/hide out... It's way easy to inspect your chicken's skin when their soaking wet...
Then, they go back outside. 
I have found out that chickens DON'T poop when you hold them--I kid you not--it's a known behavior in chicken showing ... I HAVE had one hen poop as soon as I put her into the bath, but that time, I was lucky--I hadn't put all the bins in--I was just letting her soak in warm water because we all panicked that she was egg bound--she wasn't, but she sure pooped big time. I just wiped out as much as I could, and double wrapped it into grocery bags and threw away outside--it DOES stink!
Another thing that I've learned about what to apply to ITCHY birbs, is you could put in like a cup of aloe vera juice into a squirt bottle, also with warm water, and spray it onto them--if they come to you or let you hold them, OR, you could have aloe vera juice handy when you're about to pull them out of the rinse bath, and pour it on them (also with warm water mixed in)... totally safe, and works really well as an anti-itch surfactant.