I linked in the guide in your other thread but I’ll link it again now. (I will also merge this with your other thread to keep the same issue all to the same thread)
I would go to the vet now to be honest and get a proper diagnosis.
Make sure to tell the vet you’ve already given a pet shop treatment, the dose you gave and the date upon which it was given.
Mites are not the only cause of dandruff/flaky skin so this is why treating on spec without official vet diagnosis is never a good idea.
The mites will not infest you or your home. They only want the piggies.
Use F10 to disinfect the cage a few days after each treatment.
You don’t need to clean daily - the mites will die following the correct strength treatment and fall off piggy. The clean a couple of days after treatment will get rid of anything in the bedding. You are then waiting for the next round of eggs to hatch, give the next treatment, wait a few days then do an F10 clean etc
Wooden hides and toys can be soaked for a few hours in the F10 solution, left to dry and then returned to the piggies after the full treatment is complete in six weeks time (either that or will you need to be soaking them after each treatment also). You can use cardboard items in the meantime as they can be thrown away after each treatment.
New Guinea Pig Problems: Sexing & Pregnancy; URI, Ringworm & Parasites; Vet Checks & Customer Rights