What are you using to measure humidity? If it's a dial type gauge stuck on the side, those are often wrong. I use an Accurite Indoor/Outdoor thermometer from Walmart ($12) that measure the two temps and humidity. I keep humidity between 50-60% all the time, then when I see the cloudy cast to the belly, I mist, sometimes several times a day, to keep humidity 70-75%. I've never had a problem with sheds, even on the rescue BP that the girl had for two years and "never had a normal shed". His first shed (and all the rest) with me using my normal method was absolutely perfect.

Also, are you soaking your snake before he sheds? That's actually a bad thing, as it pulls natural oils out of the skin that help them shed, and can make a shed go badly. One should only soak after a bad shed to help remove it.