Quick answer
Some people take in the instruction and do nothing with it. The verse observes the gap between comprehension and response.
Read both lines
The second line is the sharp one and it forecloses the obvious excuse. Though he understands. The problem is not that the message failed to land, was unclear, or went over his head. It arrived, and nothing followed.
The verse belongs beside 29:15, which paired rod and rebuke, and beside 27:22, where grinding a fool changes nothing. Together they describe the limits of correction honestly from three angles. This one is uncomfortable to sit with, because it names a situation in which more explaining is not the answer and the book does not supply what is. It is describing a servant in an ancient household, not licensing anyone to move from words to force, and 29:21 in this same chapter will warn about the other extreme.
Related: Proverbs 29:15 · Proverbs 27:22 · Proverbs 26:3 · Proverbs 29:21
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.