It is not unusual for some students to find lessons boring or overwhelming. Try positive reinforcement. Don’t give empty praises but do take note of any progress made and acknowledge it, no matter how small.
If you made any agreements with your students about their homework (e.g., you will allow snacks for everyone if they come with lessons prepared), then make sure you follow through on your agreement.
Finally, be positive and supportive. Teens respond not just to the work but to the person conducting the lessons. They will be more open and cooperative if they know that they can trust you.