How long is the lesson? Sometimes this happens with long lessons and small groups or individuals. I
teach English in Italy, and I have had this happen with a few students. Usually, they are tired or "burnt-out" on studying as they have so many extracurricular activities. Try to do a quick survey about their interests in written form- "disguise" it as a job application, where they have to list their strengths, weaknesses, hobbies and commitments and ideal working environment. It should give you some clues to their personalities and interests. You can build a lot on a student's profile.