Piling on criticism upon criticism doesn’t work, because you’re forcing the other person to defend their honor. Criticism only works if it’s used sparingly and the other person doesn’t feel like you’re devaluing them as a person.
Also, you can’t teach what the student isn’t willing to learn. It’s also possible that they don’t want you to be their teacher. For the most part, we don’t like being preached to, but we do like the idea of other people being preached to.