This is a question often asked. The arm of Omnipotence is not shortened that it cannot save. Thousands have been restored in mind and body in answer to the prayer of faith. Yet he never works unnecessary miracles. God has given us means and endowed us with intelligence to use these means, and he will not withhold his blessing upon their use when we ask it in faith. We ask him to feed us, but we must labor with our hands and not expect him to bless our idleness nor our lack of effort. So, if we ask him to heal us, we must use in faith the means he has supplied, with all the intelligence he has given us. It is simply "tempting God" to neglect his means. Jesus himself applied the clay and the spittle to the eyes of the blind. Naaman had to bathe in the Jordan. Even in the healing of the soul, which is an operation of the Holy Spirit, we must co-operate, and while he works in us, we ourselves must work with "fear and trembling." And if Divine wisdom should see fit to withhold the boon we crave in the form we ask, we must submit in faith to his will, as he knows what is best for us. Strength is often made perfect in weakness and many things we mistakenly call evils are blessings in disguise.
