I would treat again for mites and then repeat that treatment 10 days later in order to break the life cycle of the mite. It is also important to treat their housing as red poultry mite spends most of its time off the bird.
Infection of the vent area, can also lead to feather loss in this area. If the vent looks irritated or has a bad smell, this would require a trip to your vet, so that a causal agent can be isolated and appropriate treatment started.
It is important that your birds are on an age appropriate diet. Some birds will go through a partial moult at this time. Feather production requires a lot of protein. If they go out to free range make sure you feed them and they eat a good portion of their diet, before you let them out. So they are not just choosing tasty things to eat, rather than what is good for them. You can increase the protein in the diet, whilst they are replacing feathers, by adding in some dry cat food.
I would also add apple cider vinegar to their drinking water. This acts like a tonic.
And so that they get the most out of their feed add in some probiotic, or live yoghurt to boost the guts friendly bacteria.
It is also a good idea to watch their behaviour, feather picking is a nasty habit that some hens can develop. I would be suspicious of the bird with no feather loss. Sometimes this habit can be broken, if the culprit is isolated for a few days, so that she is lowered in the pecking order. Otherwise make sure that they have enough space to get away from each other, add in environmental stimulation by hanging old CDs around or fresh leafy greens for them to peck at