God only heals those who ask to be healed.
Also, it depends on what you mean by "heal."
If you mean spiritual healing, then the person has to believe they are a spiritual being and they have to "know" that God can heal them. Christ showed the way.
If you mean physical healing, then the person has to "know" that God has the power to heal them.
That "knowingness" is called "faith." If a person has any doubts, then the desired healing will fail.
Many people don't want to be healed. For the physically crippled or diseased, they cling to their infirmity as a badge of self-righteousness (ego). For the spiritual crippled (practically everyone on this planet), many don't believe in such things, and it has been said that such is the only sin that cannot be forgiven. You have to admit you have a spiritual self before it can be healed.