But let him ask in faith, with no doubting, for the one who doubts is like a wave of the sea that is driven and tossed by the wind. For that person must not suppose that he will receive anything from the Lord; he is a double-minded man, unstable in all his ways. (James 1:6-7 ESV)

 What to do:

“And Jesus answered them, “Truly, I say to you, if you have faith and do not doubt, you will not only do what has been done to the fig tree, but even if you say to this mountain, ‘Be taken up and thrown into the sea,’ it will happen. And whatever you ask in prayer, you will receive, if you have faith.” (Matt. 21:21-22 ESV)

“And whatever you ask in prayer, you will receive, if you have faith.” (Matt. 21:22 ESV)

 Quotes:

“When the devil sees a man or woman who really believes in prayer, who knows how to pray, and who really does pray, and, above all, when he sees a whole church on its face before God in prayer, he trembles as much as he ever did, for he knows that his day in that church or community is at an end.” - R.A. Torrey

“Do not strive in your own strength; cast yourself at the feet of the Lord Jesus, and wait upon Him in the sure confidence that He is with you, and works in you. Strive in prayer; let faith fill your heart-so will you be strong in the Lord, and in the power of His might.” - Andrew Murray