God cannot hear the prayers on our lips often because the desires of our heart after the world cry out to Him much more strongly and loudly than the our desires for Him.
Andrew MurrayThe Lord gave the wonderful promise of the free use of His name with the Father in conjunction with doing His works. The disciple who lives only for Jesus' work and kingdom, for His will and honor, will be given the power to appropriate the promise. Anyone grasping the promise only when he wants something very special for himself will be disappointed, because he is making Jesus the servant of his own comfort. But whoever wants to pray the effective prayer of faith because he needs it for the work of the Master will learn it, because he has made himself the servant of his Lord's interests.
Andrew Murray