Hey,
Some animals just don't do so well, in modest to moderate, humidity. I find that some STP's, and Bloods, might actually do better if they are kept at the higher end of the humidity chart (70-80%). Try replacing the cage substrate with Cypress mulch at about the time that he or she is going to shed and once it is shed out you can return it to it's normal substrate (newspaper) etc.
If you don't use the method described you might want to close off the top of the screen with some kind of plastic or other material that keeps the humidity from escaping and evaporating (a plastic garbage bag works great). Also, be sure to provide the snake with a large enough water bowl if you have a large enough enclosure to allow for it to soak in. I realize that there isn't enough enclosure space with some set ups but this method works if granted and made available to them.
Also, make sure that the snake is drinking sufficient water, as they some times go with out drinking just prior to shedding and actually dehydrate and this can be a cause for the dry and adhered sheds.