I find Repti-bark to be a terrible substrate for ball pythons. It's too dusty, and leaves little black chunks everywhere that I always get tricked into thinking is mites. Not to mention, it cannot possibly be very comfortable for them. Try switching to aspen (probably the best choice) or Eco-Earth.
Also, make sure you're using belly heat with a good thermostat, and NO heat bulbs or ceramic heat emitters.
The ideal suggestion would be to put your ball in a tub instead of a tank, as they're too difficult to maintain the proper conditions. In a tank, something is usually sacrificed, be it the proper heat, or the proper humidity.
If you don't want to use a tub, try an even bigger water bowl, and maybe put a tiny bit of the water bowl overtop of your under tank heater.