I don't want to sound like a psychology textbook, but have you tried to build a rapport with some of your students? This has helped me tremendously with my 3rd grade middle schoolers. When I walk into the 1st or 2nd grader's classroom, they instantly love whatever I do, but the 3rd graders could easily care less. And I don't necessarily blame them. They are so over-worked and pressured from their parents and teachers to score high on their exams. Middle school in Korea is a transition in which they become fun loving learners to "rote memory test learners" By the time I show up, it's finally a period they don't have to pay attention. At least at my school, my class isn't really counted for in their grades. In exception for a few classes where the teacher marks a + or - for participation (and that does help too!)
But really what has helped me the most with the older kids is sitting down and developing some relationships with a few of them. When my lessons become a little drab, I can just give them a look of dissapointment and most of the time they want my acceptance, so they'll perk up.