1. "No" is an answer also.
God does not always give us what we want; He gives us what we need.
2. We do not understand the way in which God responds to our requests.
He may not grant an immediate answer, and sometimes our prayers are answered in a way we fail to recognize.
3. Our motive for making a request to God must be pure.
If your prayer is self-centered, concerned with the gratification of your own desires, God cannot, in faithfulness, grant your request.
4. God has given man free will.
If your prayers are not always answered as you expect, it may be that the person for whom you are praying has not yet responded to God's call.
Read Billy Graham's full response here.