If its mites you should be able to see them. They are small and black dots that will be crawling on it. You should be able to find some in the water bowl since it has been soaking. Look real close because they are very small but visible to the naked eye.
If you find them you are going to need to clean everything and get rid of any substrate you have been using. I like Reptile Spray and its sold at both Petco and Petsmart. It is safe to use on the snake and can be used to treat the cage, hides, bowls, and the surrounding area of the cage. Follow the direction and clean everything very well. Then put the snake back in the cage on paper towels. The paper towels will make it easy to see any dead mites after treatment.
You can even spray the floor around the cage and then vacuum it.
If it is not mites your temps may be too warm.