Why won't my ball python shed correctly?

I have had my ball python for about a year now, and every time he sheds his skin comes off in small patches, and a lot of the time does not come off at all. When I assist him in shedding, I let him soak in warm water for a while then I pull the skin off, but I have heard you are not supposed to pull it. What should I do? I have tried raising the humidity to around 70-75% when he is shedding and I also covered part of the lid to hold the moisture in. There is a large water bowl in my snake's cage, and I have seen him soak in it but it never helps the shed. I am using an aspen bedding, should I use something different? Please help