I've taught many people. There is always at least one person in every group who is fully capable of doing what I am instructing them to do, but one thing is stopping them: belief in themselves.
When an instructor is teaching, they are placed in a position to give their students everything: A structured class, choreography, correction, and praise. They are also giving them or denying them validation.
Words are unneeded. A glance; a hand gesture; a certain tone of voice; the smallest change lets a student know whether or not they are approved. From that information, they draw their confidence.
Don't tell them to believe in themselves or work harder or do it better.
Believe in them. Approve of them. Love them.
They will know whether you do or not. And when you do? That is when they will get better.
Live In Motion,
-Joél Casanova