Answer: The most common reasons for lilacs not to bloom are lack of sun (they need full sun all day long) and improperly timed pruning. Any pruning should be done in the spring right after their bloom season. Lilacs bloom on wood that grew the previous year, so pruning in summer, fall, winter, or early spring will remove flowering wood.
The plant that is only 16 inches high sounds as though it may be stunted, perhaps due to a rooting problem. Lilacs should be able to grow at least a foot or more every year when they are young. You might want to look into that, perhaps in consultation with your county extension.
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