Staff makes a huge difference. Colts were brought up; they pushed into finals for the first time in recent years after having had consistent staff and membership loyal to that staff for several years prior. Then, key staffers leave and the corps floats back to where it was. Now, some of those key staffers are back and the Colts are climbing again. Coincidence? Not likely.
You've got to build a culture that keeps membership around longer and prevents the top individuals from migrating upwards. If a corps like Colts could keep all their talent from wanting a different experience from different teachers, they'd have a much more talented corps. If you could bring those top names to them, they might stay. But for many who leave, it's not about placement, it's about learning from the best in the activity wherever they might be at the time. Look at the influx of talent to BKs drumline when Hardimon was there; the BK corps wasn't contending for titles but the best always want to learn more and learn from who they perceive to be the best. That's often misconstrued as "ring chasing" when it's a personal choice to expand their future goals.
I just inserted Colts in there - you could argue the same for any corps in that grouping just about.