I'm no expert when it comes to snakes and their cages, but it would probably help others who are the experts to know what kind of heating you are using.
It could be that you need more time to get the tank going before you get a snake. It may need to be sprayed everyday, maybe multiple times, to build up humidity. Yourcage may need to be sealed better to hold in the humidity. It could be your substrate is still too dry.