Reptile Relief if that is what you are using is not going to kill the mites in the cage. You need to treat the cage as well. The one time i had to deal with mites, I used Provent-A-Mite. Throw the substrate out. use paper towels. Put all the decorations except the hides into a ziplock bag and shoot a shot of PAM into it, seal it and put it in the garage for a couple months. Then every week, soak the snake in a little tub with a drop of dawn dish soap in it to break water tension so the mites drown. While you do that, change out the paper towels and reapply the PAM as per directions. Leave the water bowl out overnight on each night you apply the PAM. Do this weekly for about 2 months. Also spray PAM on a rag and wipe down the outside of the cage especially cables that go into the cage and vent holes as well as the top and bottom rims of the cage if its glass tanks.

After a couple months you should be fine. I still wipe down the bases on my cage stacks once every couple months as a preventative in case i bring home a hitchhiker.