It sounds like the trees are likely never really getting established well. A couple thoughts on what it could be…..

It might be your soil giving you issues. I think pears prefer a sandy, loamy type soil versus any harder clays. It could be the quality of the trees you’re getting and how well they’ve been taken care of prior to you getting them. It also could be the size of the trees you’re trying to transplant. I’ve found it much easier to establish younger, smaller trees.

I think the first thing I’d attempt to change is I would try to order some smaller bare root seedlings from a reputable nursery (not saying you’re not using one now). Try to use one close to home if possible. Ideally, you would like to know that the tree has not originated from a nursery in a different climate. I think its something like 200 miles north or south at the most from where you're planting is about what you want to shoot for. That's what we were taught with pine trees anyways.