Answer: The most common reasons for lack of bloom on lilacs are being grown in too little sun and pruning at the wrong time.
Lilacs do best in full direct sun all day long or for a minimum of six hours including noontime. In shady locations they will grow lush foliage but blooming is greatly reduced or nonexistent.
Lilacs bloom on wood that grew the year before, so the only "safe" time to prune is in early summer right after they bloom (or would bloom.) Pruning in late summer, fall, winter, or spring will remove flowering wood and flower buds.
The other possible reason for lack of bloom can be overfertilizing, especially with nitrogen. If for example they are growing in a lawn area where the lawn is heavily fertilized, this could be excessive -- especially if the lilacs are also fed independently. In many cases, an annual top dressing of good quality compost along with an organic mulch layer year round would be adequate.
I hope this helps you trouble shoot.
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