Imaam Ibn al-Qayyim (may Allah have mercy on him) said:
du`as and taawwudhaat [prayers seeking refuge with Allah] are like a weapon, and a weapon is only as good as the person who is using it; it is not merely the matter of how sharp it is. If the weapon is perfect and free of faults, and the arm of the person using it is strong, and there is nothing stopping him, then he can lay waste the enemy. But if any of these three features is lacking, then the effect will be lacking accordingly.
(al-Daa wal-Dawaa, p. 35).