Have you ever noticed that if you pray for an unknown country or unknown people not much or nothing may happen. But if you pray ferverently or passionately for a friend or child then quite often something happens. Why?
It's an interesting thing, that if you ask with right motives then you will receive. What does that mean? What are the right motives? John chapters 14-17 makes it clear – Jesus is all about loving others. Most of His miracles were motivated by His compassion for others – that's the right motivation to have behind asking for something.
If you ask for something (like the healing of another person) out of compassion for that person (not for your own ego) then your prayer will be answered. Love others – that's His Great Commandment.