Answer: It sounds like the shrub might be a candidate for some renewal pruning to encourage vigorous new growth. Lilacs bloom on "old wood" meaning the previous year's growth, so do any pruning in the spring immediately after they bloom (or in early spring before they bloom if you don't mind sacrificing the current year's flowers.)
You'll need up to three years for this
technique. As soon as they have finished flowering, take your
pruning saw and cut down 1/3rd of the bush. Cut the oldest,
and fattest trunks as close to the ground as you can
comfortably get. You'll get some new wood this year. Then
next year after it blooms, do the same thing. You'll get more
new wood after that. The third year, cut out the remaining old
part of the bush and VOILA! you'll have an entirely new
shrub, growing on old, strong roots. It should reward you
mightily. Not only that, all the flowers will be down where
you can enjoy smelling and cutting them because they'll be
You actually could do this in halves over only two years...or if you're in a hurry, in one. Just cut it back to the ground. It should sprout and bloom the next year, but will look pretty sad in the meantime!
You could also try giving it a good helping of compost or aged manure in late fall or very early spring. Keep in mind too that most older lilacs tend to bloom on an alternating schedule, heavy blooms one year and lighter the next so there will be a certain amount of variation in the bloom performance no matter what you do.
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