The Bible offers much help in understanding God's ability and purposes for healing. First, it is clear that God can heal in any way and at any time He desires. He can heal instantly (Matthew 8:13) or after a period of time (Mark 5:5; a woman praying for twelve years).
Healing - What does the Bible have to say?
Second, the Bible reveals that God sometimes chooses not to heal in this life. For example, when Jesus was healing people in Capernaum, we read in Luke 4:40-43:
Now when the sun was setting, all those who had any who were sick with various diseases brought them to him, and he laid his hands on every one of them and healed them. And demons also came out of many, crying, "You are the Son of God!" But he rebuked them and would not allow them to speak, because they knew that he was the Christ.
And when it was day, he departed and went into a desolate place. And the people sought him and came to him, and would have kept him from leaving them, but he said to them, "I must preach the good news of the kingdom of God to the other towns as well; for I was sent for this purpose."
Jesus healed many with sicknesses in this account, but left others who had not yet received healing. While we may not know all of His reasons, it is evident God sometimes has purposes for a person's life that includes not healing a particular problem at the time or in the way in which we would choose.
Why would God sometimes not heal a person? Sometimes God allows a person to suffer for the name of Christ. Paul's calling included this aspect of suffering: "I will show him how much he must suffer for the sake of my name" (Acts 9:16). At other times, sickness simply occurs as part of the imperfect human condition. Bodies age; bones wear down. At still other times, God allows sickness as a form of judgment. For example, the Corinthian believers who took the Lord's Supper with irreverence were judged with physical sickness and even death (1 Corinthians 11:30).
Does God still heal supernaturally? Many modern accounts seem to indicate He does, though caution should be used in evaluating such claims. Further, God's ability to heal supernaturally does not negate the important role of obtaining appropriate medical help during times of need. Jesus did not speak against those who sought help from doctors (Mark 5:26) and encouraged medical assistance at times (Luke 10:34).
In summary, God can and often does heal. Yet sometimes God chooses to heal slowly or even not all in this life, often for reasons not understood at the time. Regardless of how God chooses to act, we can trust He is perfect in all His ways and knows the best way to respond to all those who turn to Him for help in their time of need.
Why does God allow sickness?
Is sickness ever part of God's will for believers?
Can Christians feel disappointment with God? Is it wrong to experience disappointment with God?
Is it okay to question God?
Seeking God - Why is it important?
Truth about Everything Else