Working with clients over the years, I see two comprehensive reasons why a Being might not heal.
1. A person who doesn't want to heal. The "story" of illness is commonplace in this society. Some people are addicted to the emotion. Some people are addicted to the attention being ill gives. Some people think they are ready to heal, but haven't developed the self-awareness to truly understand.
2. A person who won't or can't devote the energy required to heal. If someone is facing disease their energy body is already weakened. Some people run to healers to do the work for them. Some keep their schedule so full that healing will not fully take place.
There is a huge amount of energy required to heal the body on every level-spiritual, emotional, mental, energetic and physical. When I recovered from the tumor being taken out of my head, I was back in sync after 10 weeks. My primary focus was to rest and allow for healing.
I leave you with a bit of humor:
If you think time heals everything, try sitting in a doctor's office.