I’ve heard lilacs love/need a lot of calcium.
You might try buying a bag of lime and spreading a couple pounds around the base.
Will try DJF. I will do the manure first.. wait a few weeks and then the calcium. Even if I don’t see an improvement this year... perhaps next. I’ve seen homes that have giant Lilac bushes (darn near look like small trees) and full of blooms. Lucky people!