Many kids today simply don't know how to fail. They're always successful somehow - or they get to "win" anyway. Failure is not an option - and not in a good way.

Fail your STAAR? You still move up a grade.

Fail a class? It's the teacher's fault. You still get to move on.

Don't score a goal? The team goes out for pizza/ice cream/etc afterwards anyway.

Honestly, you beat this by introducing as much healthy competition as you can. Start having weekly chair tests even as far down as beginner band. You have to teach them the value of work, not just work. You'll lose a few - but that's why band is such a cult of personality for the director. You'll gain as many back if you make kids want to be in your class.