If an individual student is not responding to a certain training style, try something else. Remember, positive reinforcement for the trainer has lots of collateral effects. First of all, your students will learn faster and be more successful. Second of all, their dogs will learn faster. If the handler is uncertain, discouraged, depressed or intimidated it will most certainly reflect on their relationship with their dog.
Dogs respond much better to a handler who is happy, confident, secure and enthusiastic.
Finally, if you are nice to your students, they will love you. Remember that unlike dogs, we humans are starved for a kind word. Think about it. We are highly motivated to train our dogs. BUT it doesn't relieve us from the responsibility of using humane teaching methods, and it doesn't negate the fact that if we are positive, our students will be much more likely to succeed.