That's a hard question without knowing more about it. Generally, pruning is a problem for lilacs. They should only be pruned in the late spring right after flowering. Some varieties do take their time blooming. I had three that didn't bloom for five years and now bloom every year without problems. If they are overfed they will also not bloom well because they'll use the nitrogen to develop foliage and not flowers. If you give them sunlight, don't prune at the wrong time, and refrain from feeding, all you can really do is be patient. Older varieties, in particular, can be stubborn bloomers.
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