I think it is pretty common to have a hard day at work or school then drag yourself to class ... as a student. This does not change when you are an instructor, but you have to generate the right energy for the class.
Sports psychologist say that students remember what it feels like at the end of class. If this is worn out, dog tired, or hurt ... that is what they feel and associate with the workout, regardless of what other good stuff you did prior to this. The intellect knows better, but the "feeling" or kinesthetic experience is what the body "remembers".
Try something a little different: Start out warming up, semi-aerobic to full aerobic (trainer phase), taper off during the "teaching phase" of the class doing techniques, forms, sets, weapons, etc. , then pick the pace up again just the last 5 minutes of class. Kicking shield drills with knees and elbows, just a couple of times across the room. Or maybe a real intense 5 minuted drill of "ripping" off techniques. Push a specific set, right side, left side, right side, left side aerobically.
Just so you realize whatever they feel at the end of class is what they walk away with. They remember the intensity at the end of class. Do this a couple of times a week and see if there are any comments about how good a class it was ... even if it was so-so, the end often matters more to the students than we, as instructors, are aware of. We also feel better after those kind of workouts.
-Michael
UKS-Texas