After referring to Isaiah 55:10-11 Pst John Piper in his book "Supremacy of God in preaching"  says this about fruit in our preaching, "You can mark it down: God will hide from you much of your fruit.  You will see enough to be assured of His blessing, but not so much as to think you could live without it.  For God aims to exalt himself, not the preacher, in this affair of preaching."