Are you using a UTH with the lamps? The lamps are killing the humidity, if you're using a regular terrarium with a screen top my advice would be to cover the majority of the cage with a towel. If you need the lights because they're your only heat source then you need to cover as much of the cage as possible to keep the humidity up. You can always keep one light on one side of the cage so the snake has a hot side and cool side to thermoregulate.