HS Directors should visit the MS a lot; agreed 100%! But we have classes to teach too. If the only MS Ensemble is scheduled during my class, how exactly am I supposed to come and recruit? If you don't give me an opportunity to pull kids, help, and build relationships? If COVID protocols are that I can only come into contact with students during the school day, and I can't recruit after school?
On years like this, it takes a team to have a successful band cluster. Bad retention from MS to HS but good retention within the MS does not necessarily mean the MS director is doing their job. Is the culture consistent from MS to HS or does the HS have significantly more rigorous expectations of behavior? That can cause kids to drop. There are so many factors to cause attrition between grade levels. I've taught MS, HS, and a situation where I did both - it is not and cannot be 100% incumbent on the HS directors to retain students from MS.