Scaley leg mite infestation can be deep in the leg/skin tissues. In severe cases, it can cause toes to fall off. In severe cases, you would need to repeat the treatment, and it would probably be a good idea to use some sort of pestcide. What I have done for scaley leg mite is to soak the chickens feet and legs in warm water and epsom salts. Do not let the chicken drink the water. Soak at least 15 minutes to soften the skin. (20-30 minutes is better, but if you have a lot of chickens to do 15 is adaquate) Make sure to treat every chicken, even those without external symptoms of scaley leg mite. You can gently scrub the chickens legs with a soft brush, but not too much! Very gently!!!! Then, mix vasaline with a good amount of poultry sust or sevin dust. Put on rubber/surgical gloves, and rub the mixture thouroughly into the chickens legs. Work it under all the riased scales, toenail beds, in between. You really need to get it in there. You should repeat this weekly in severely infested chickens until you are certain they have no mites. I also routinely spray my chicken house and roosts with a flea/tick/yard spray that you attach to a hose every month or so. When you do it, get every crevice. If you can't spray the whole house, spray the roosts.
Also, please post pictures of the chickens feet so people with experience can verify if it is indeed scaley leg mite your chickens have.