My MS band which normally would have 40-45 kids on an average year dropped to 30. I was already in "panic mode" but then an additional 4 kids left after the first day of school and 2 more after the first semester. That first week of school I was reflecting hard on what I was doing or not doing to make so many kids leave.
By the spring I ended up taking 23 students to UIL C/SR which on paper looks awful but they all really stepped up and did the job. As much as we like to have numbers sometimes its best to have a smaller band of the kids who want to be there, who love band and will bring that attitude to the program. No matter what we do right, some kids are going to leave. Stick to your process and vision for the program and the students you have will rise to the occasion.