Answer: The most common reasons for lack of bloom on lilacs are lack of sunlight and pruning at the wrong time. (They do best in full direct sun for a minimum of six hours a day including the hour of noon.)
This shrub blooms on old wood, so the best time to prune is in late spring right after it blooms. If you prune it in early spring or fall you will be removing the flowering wood. A new shrub would not need much pruning, but after six years you could probably begin to thin it out a bit each spring -- but do this after it has bloomed for the year.
Remove a few of the oldest stems or branches by cutting them off at the bottom near the ground. Also remove the weakest or thinnest new shoots coming up from the ground; take out anything smaller in diameter than a pencil. If you do this each year you will have an ongoing process of renewal and encourage vigorous new growth each season.
I hope this helps!
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