Well lilacs don't bloom if you've pruned them in the spring or if they are not getting enough sunlight. Did you prune them at all? Lilacs can be pruned, BUT it has to be done right after they bloom. If you cut them back in the fall, winter, or early spring, you'll destroy the spring flowers. One other possibility might be that they are being overfed. If you give them too much nitrogen they will grow leaves and not focus on flowers.