Something must be wrong in the soil. I suggest you check for poor drainage or soggy conditions as well as soil that is too dry. If the soil is a black clay the planting hole can become an underground container restricting roots from moving out into the soil and thus causing a major loss of vigor. Digging down in several places around the root ball to loosen the soil and mix the transition zone between the right potting mix from the original container and the surrounding soil may be helpful. A more drastic option would be to dig up the plant, loosen and mix up the soil with a spading fork and then immediately replant it and water it in well. Lilacs hate the heat of the south, and although the Lavendar Lady is the best of them for warmer climates it still can struggle in summer.
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