The most powerful prayer I have read in the Bible was one where it says, "God repented....."

In Exodus 32:11-13, Moses prayed to God on behalf of the children of Israel.
1. He spoke to God of how this action would not glorify His name. It would be contrary to His ways.
2. He spoke to God of His promises, that this would be contrary to his will, his Logos.

A prisoner today makes appeals to a judge in this same way. He studies past precedents of this judge. If one appeal doesn't work, he crafts the next appeal on different criteria, trying to find that one combination that can win a reprieve.

So it was with the widow before the unjust judge in Luke 18:1-7. She certainly didn't repeat her appeal with the same criteria each time or she would have been kicked out of court. She became familiar with the judge and came up with some new angle each time. He had to listen because each appeal was a little different. So it is with us. We must get to know God's will and ways so we can ask according to His will and receive an answer. (1 John 5:14)

No answer from God is not the same as hearing "no". When I pray for someone and nothing happens, I will try another promise -- deliverance. Sometimes I get an immediate answer; sometimes not. If not, I will try inner healing, to see if the person's emotions or volition are the obstacle. Sometimes I get an immediate answer; sometimes not. If not, I will try breaking any generational curses, to see if a family curse is the obstacle. Sometimes I get an immediate answer; sometimes not. And so on.

When the gift of faith descends, Jackpot! The client is healed.