Hello Brian,

Thank you for your question. I am sorry that it involves the loss of some of your plants.
I am including some helpful websites here that offer possible explanations and solutions. I am guessing that your lilacs might be reacting to the heavier rains and warmer winters we have been experiencing in Ohio. A common cause of lilac problems is soil that is too wet and weather that is too warm and humid. If there are trees nearby that have grown to shade the lilacs since they were planted, that might also be an issue, as the lilacs might not be getting enough sun.

https://extension.umd.edu/resource/lilac-identify-and-manage-problems/

https://www.gardenerreport.com/why-is-my-lilac-dying-how-to-save-it/

https://extension.psu.edu/lilac-diseases

I hope these will be helpful and that you will be able to save your remaining plants.
Meanwhile, you might consider trying to propagate some new plants from your existing ones. it is a way to preserve the lineage.

https://selfgardener.com/propagate-lilac/

Best wishes,