And there came a leper to him, beseeching him, and kneeling down to him, and saying to him, If you will, you can make me clean. (Mark 1:40)
Healing the sick was only one of the miracles Jesus showed us. He is our greatest physician. When we pray for the sick, sometimes we ask for what is not in God's will. It's very hard for those outside of his will to understand his will. Sometimes the answer is "no".
When the leper knelt before Jesus, he already believe that he could be healed by him. He had faith that if it was our Lord's desire to, he WOULD heal him. Many times we bring our petitions to him without truly believing that he can do all things. Our prayer should reflect the "would" and not the "could" of any request. There is nothing our God cannot do!
When my children ask me to pray for one of their friends, it is in that request that I see the hand of God at work. With each request, they take one step closer to knowing Jesus. The fact that they have asked for prayers tells me that they do believe. But believing, as we know through the scripture, is not enough. Even Satan believes in God!
God does not put conditions on the healings granted. He doesn't charge for his services; they, too, are gifts of love and mercy from him to us. He also does not deny them because of something we've done or failed to do - they are denied, again, because it is his will. In that, you may ask: "why, then, should I pray - God already knows what the outcome will be and he's already decided the fate of the one I am praying for?" We pray because it is what we are asked to do. We are to petition him with our requests. We are to "pray for one another" (James 5:16), we are to "pray without ceasing" (1 Thes 5:17), we are to "pray with our spirit and our mind" (1 Cor 14:15) and we are to pray to our Heavenly Father, through Christ alone! He alone is our intercessor! It just may be he's waiting for YOUR prayer!
Present your request to him and know that the "could you" part is a given. He can do anything. Make it a request, by faith in him, and know that what ever the outcome is, it is his will.