Are these the 5+ year old trees that weren’t getting enough light? I’ve noticed that most of my new and 2nd year trees have very few fruit set. Maybe 1-2 per tree, if that, with 0 on many, especially the new ones. My So and some of the grafts on it (like Honey Jar) again have a decent set. I bet that most of it is the fact that the tree is in year #6, but part of it could be the 15+ varieties grafted on the same tree. That’s got to help pollination. Maybe you need to try the paint-brush approach for a few days and see if it makes a difference. I may do that on some of the 2nd year trees.
I am excited to try some of the new varieties which have 1-2 fruits, such as Sherwood, Masandra, Tigertooth, and a few others.
Tony is still way ahead of me in terms of fruit size though. I bet that some of my flowers are still setting.