Then I leave it alone. I won't badger a kid, one talk and that's it. If they don't show up, I try not to get heartbroken about it (even though, it still stings after 17 year).
And like others, I try to maintain a good relationship with those kids. We're at a small school, and any bad relationships can be detrimental. If non band kids can like and respect the band, it makes it much easier to keep some of those on the fence kids.