Only God can make our words do His will.
So Samuel grew, and the Lord was with him and let none of his words fall to the ground.
The Lord did not allow any of Samuel's words to fall to the ground. The image recalls Onan, spilling his seed on the ground and wasting what God intended should be fruitful. Samuel's words were fruitful – all of them. With Samuel there were no wasted words. All the people of his day recognized him as a prophet because of the fecundity of his words.
Three conditions attended to this result. First, Samuel grew. He grew above all, we may assume, in his relationship with the Lord and understanding of His Word (cf. v. 7). Second, the Lord was with him. He maintained himself as one who walked with God, seeking His promises, resting in His strength and provision, manifesting His holiness, and communing continually with Him. Finally, Samuel spoke. We can believe his words were always measured, framed within and according to his understanding of God's Word, never frivolous or vulgar, and always cast in the light of God’s covenant and Law.
And he must have spoken in such a way, as do God’s promises and Law, as to hold out blessings and prescribe courses of action.
Only God can make our words count. We need to seek Him as we are preparing them, while we’re delivering them, and in following-up after them.