How can God choose not to heal someone when He already purchased their healing? Was His blood enough for all sin, or just certain sins? Were the stripes He bore only for certain illnesses, or certain seasons of time? When He bore stripes in His body He made a payment for our miracle. He already decided to heal. You can't decide not to buy something after you've already bought it.
There are no deficiencies on His end - neither the covenant is deficient, nor His compassion or promises. All lack is on our end of the equation. The only time someone wasn't healed in the Bible (gospels) is when the disciples prayed for them. For example, Mark 9 when they prayed for the tormented child. They did not have breakthrough. But then, Jesus came and brought healing and deliverance to the child.
Jesus Christ is perfect theology - He is the will of God. We can't lower the standard of scripture to our level of experience . . . or in most cases, inexperience. It's a very uncomfortable realization - not everyone can handle it. Most create doctrine that you can't find in the person of Jesus. He is the will of God.
How do we fix the problem?
1. Realize it's not God's fault.
2. Learn from others who see miracles. Have them pray for you. If you don't know people who see miracles, find them. Books will help, if the author has a miracle lifestyle. (Don't try to learn from those who only have the theory of miracles.)
3. Do what Jesus told His disciples to do when He addressed the problem - pray and fast (not just for a specific problem, but for a lifestyle anointing.)
4. Take risk - pray for people (NOT - "if it be thy will" kind of prayer. In the thousands of people I've seen healed, I've never seen anyone healed from that kind of prayer.)
5. It's hard to hurt someone's faith by praying for a miracle. Not praying for a miracle gives them no chance for increased faith. That is what hurts faith. The real goal is to show people God's love. And faith grows in the display of God's love. His love is seen when we show compassion and display God's power.
6. When God heals people, give Him thanks and praise. Learn from it so it will happen again.
7. If someone isn't healed, realize the problem isn't God, and seek Him for direction as well as personal breakthrough (greater anointing for consistency in healing). Also, don't take it personal. There are other factors involved besides great faith. That is only one element in the equation. Just learn to do your best to be faithful to His gospel, and honor Him for the results. It's also not wise to blame the person who is sick.
8. There's a difference between a miracle and healing. Miracles happen in an instant and healing happens over time. It is important to recognize the progress of what God is doing in a person's body and give thanks, because healing increases in an atmosphere of thankfulness.