Right, that is where you could be going wrong. The mites will not live on the girls. They live in the hen house and come out at night to feed (on the girls' blood) but don't actually live on the hens so you won't actually see the mites on the girls. Wait til dark and then run a piece of kitchen towel under the perches - if you get red spots, you've got red mite. Another way of checking is to look at the end of the perches - if you see an ash like substance at the ends there are red mite present.
Mine are perfectly clean and white, but I still had a serious RM infestation in the hen house. Wash hen house down weekly with Poultry Shield, and then sprinkle Diatomateous Earth in hen house and in their dust baths. Treat the girls with red mite powder. Also put vaseline at the end of the perches and any joints in the hen house.
My girls went off lay before I realised we had got mites - once I had dealt with the problem and treated them they were fine. This chicken keeping lark is a steep learning curve!