George Jeffreys (28 February 1889–26 January 1962) was one of greatest evangelists of the 20th Century, and who had a powerful healing ministry that spread across the United Kingdom. In his book Healing Rays, Jeffreys attempted to address the question of why not all people were healed at his healing services.
Why are some of the people who seek healing at your revival and healing campaigns not healed?
There are many reasons such as are mentioned in the next answer. All were not healed in the days of the Lord.
And he could there do no mighty work, save that he laid his hands upon a few sick folk, and healed them. And he marvelled because of their unbelief. And he went round about the villages, teaching. (Mark 6:5-6)
Do you attribute failure on the part of those who are not healed to lack of faith?
We do not attribute all failures to the lack of faith. There might be other reasons why people are not healed. Hindrances in the lives of seekers, and unwillingness to obey the commandments of the Lord, can hinder the work of healing. Again some are allowed to suffer for disciplinary and other purposes.