Samuel grew, and Yahweh was with him, and did let none of his words fall to the ground.
"All Israel knew that Samuel was established as a prophet of the Lord"  (1 Samuel 3:20).  This is a statement of what eventually came to pass and does not mean that this widespread recognition of Samuel as a prophet of the Lord was an instantaneous result.  In this book, somewhat later, Saul was apparently ignorant of the fact that Samuel was a prophet of the Lord  (1 Samuel 9:5,6).