No one knows why, but we don't always get healed when we pray. Sometimes we do and sometimes we don't. That really is God's business. He did tell us to do that, remember. No one can tell you why either.
Demons work on legal grounds most of the time.
If you were in the occult at one time, this would be legal grounds. You can command them to leave all day, but they don't have to until the occult is renounced by the person. Most demons come out immediately. We spent four hours one time on ONE demon before IT left. Sometimes the demons will not leave until you find out what their legal grounds are.