Interesting that this discussion moved away from specifically about amputees not getting their miracles to miracles in general. I am in no doubt that God wants to and does miraculously heal. I'm seeing it every night from Florida, I see it regularly where I volunteer (Healing Rooms: see
http://www.healingrooms.com/), and it happens in churches and conferences/large meetings all the time.
As to the who, the why, and the how: I don't know. God heals, not the person praying nor the person receiving. It ususally requires faith, but sometimes He just heals sovereignly. Always, though, it's to direct people back to God.