I have several years' experience as a professional tutor, and have worked with students struggling with a subject, and with "star pupils" looking for more depth in a subject. Information has to be made personal to be relevant. Unless a subject catches your interest from the first, you only feel motivated to learn information if you need it to do something you want to do. For that reason, I take a personal approach, tailoring the lessons to the child's interests, needs and skills and developing personalized materials, and attempt to relate the information to the larger world around us.
Often, if a child is struggling with a subject, it's a matter of a few key skills that he or she missed. Help that student develop those crucial skills, and the subject becomes more accessible. I have seen my students go from struggling to the honor roll, and while that kind of progress isn't always possible, improvement is available to everyone. The key is to start where that child is - and develop a bridge to where he needs to be.
Learning is like a bus. If you see a bus pulling up to the bus stop, you'll run to catch it, if you think you can. If, however, you think the bus is too far ahead of you, you won't bother to run for it. By developing materials targeted to that one student, you can help him, or her, see that it's possible to catch that bus.