Hi Carol. Pruning might help. Lilacs set the buds for next years blooms soon after they finish blooming this year, so you'll want to prune them as soon as the few blooms that they do have are done. I belive they say, if you are pruning them down (shorter) you shouldn't do any more then a third of their height. I don't prune mine much from the top, but I do prune out some of the branches each year, to kind of think them out a little.

If you decide to prune them, do it by the 1/3 method and ONLY RIGHT after they bloom. They may not bloom then again for another year or so - they start to set blooms shortly after they bloom so pruning disturbs that a lot.

-Marie

You can always remove dead and decaying branches at any time- and I have found it a good practice to remove some of the largest oldest wood as close to the base as I can cut on my very old overgrown lilacs. It really does rejuvinate them!
I have found I get less flowers on the bushes that do not recieve enough light- over the years some mine have been crowded by overhead trees.
Best of luck to you-
Julie