Why Aren't My Prayers Answered? John Piper

Back in 1996 John Piper preached a message titled, Praying From The Fullness of The Word, in which he addresses the questions, "Why aren't my prayers being answered?" - Ed Stetzer: He says in part,

It says we may not be praying according to God's will; 1 John 5:14, "If we ask anything according to His will, He hears us."
Or it could be we have cherished sin that we will not let go from our lives; Psalm 66:18, "If I regard wickedness in my heart, the Lord will not hear."

It could be that we have man-centered and not God-centered motives; James 4:3, "You ask and do not receive, because you ask with wrong motives, so that you may spend it on your pleasures."

Or it may be that we do not believe that God will do it; Mark 11:24, "All things for which you pray and ask, believe that you have received them, and they shall be granted you."

Or it could be that God wants you to persevere, and is testing your obedience to his command in Luke 18:1, "At all times [you] ought to pray and not to lose heart."

Or it might be that God is, in fact, doing far more every time you pray than you can imagine and is daily putting in place a part of the mosaic that will in good time be the full answer to your prayer (as in Daniel 10:2,12).