The mites are super tiny and can hide under your snakes scales. Removing the mites that can be seen just works on the ones that have made it to that life stage. I doubt the substrate is the issue as any mites in it would not be the type to live on your snake. Until you start to treat your snake the mites are only going to get worse.
PAM is highly recommended but not usually available locally. You can order it but that is more time the mites dig in.
I have used Reptile Spray with great success. It is only about $8 for a small spray bottle and can be found at Petco. It, unlike PAM, can be used on your snake. Just follow the directions and keep your snake on paper towels until you rid yourself of the mites.
You can spray Reptile Spray on pretty much everything. I cleaned my snakes, cages, bowls, table, and even the carpet directly around the area with it and killed them with one treatment. If your worried about the substrate you have you can spray it too.
http://m.petco.com/product/109610/Na...sultRedirect=1