Old Testament promises and requirements were written for Old Covenant folks. For any Old Covenant promise to be applicable to New Covenant born again Christians, they would need to have been repeated and confirmed by either Jesus in the Gospels or by the Apostles in their instructions to the Christian churches.
Every promise in the Old Covenant has a requirement for that promise to be effective. Those requirements were for Old Covenant people. These people were unconverted and were not filled with the Holy Spirit.
The New Covenant brought in a complete new and different way of approaching God, and the requirements were no longer based on the Law. We are no longer under the Law, but are filled with the Holy Spirit who is guiding us into all truth. Only those parts of the Old Covenant Law that are restated under the New Covenant are applicable to born again Christians.
Concerning healing. For the New Covenant, healing is part of our redemption in Christ. It has not only been promised (which was the state in Jesus's earthly ministry when He said, "By His stripes ye are healed"), but the promise was conditional on Jesus being whipped, and as soon as He was whipped, it was by His stripes that we are healed. So, the healings that were given by Jesus were conditional on the promise that when He was whipped, those healings would be confirmed. One commentator has said that if Jesus was set free without getting a whipping, maybe all those healed people would become sick again. But as soon as Jesus was whipped, the promise was fulfilled, the conditions met, and the healings confirmed.
Now that Jesus was whipped, and after His resurrection, Peter was able to say "By His stripes we WERE healed". This means that healing is no longer promised, because the conditions have already been met. God has already bestowed healing. This is why we can lay hands on sick people and declare them healed. We don't have to ask God for something He has already made available to us through our redemption in Christ.
The reason why people are not healed is because of wrong teaching. 85 percent of healing teaching and practice in the Pentecostal/Charismatic churches does not work. This is because people are ministering healing in unbelief. They are asking God for something He has already given, making a sideshow of the healing ministry, and implying that certain men have special powers for healing, when we are merely representatives of Christ.
If we look at the ministries of John G Lake, Smith Wigglesworth, and Maria Woodworth-Etter, you will see that they ministered healing in quite a different way than modern healing ministries today. When a guy puts on a white suit, gets up on a stage as a showman, then that's all he is. You will find that 99 percent of healings in those crusades do not happen. God will not share His glory with anyone. The healing ministry is not a sideshow.
Actually, the bulk of the healing ministry is not for Christians at all. It is for unconverted people to show them that Jesus is alive and the Gospel is really true, in order to bring them to Christ. So, in my belief, healing services in most of our P/C churches is a waste of time and mostly ineffective.
We have to go back to the Gospels and see how Jesus did it. When we read those accounts, we can see clearly that Jesus conducted healing in a totally different way to what most P/C churches do.