Answer: Based on your description I am not certain what is happening with your lilac, but maybe this will help you trouble shoot.
Lilacs grow and bloom best in a full sun location with direct sun all day long. They can tolerate a half day of shade and still bloom. If your plant is growing in a shady site, that might account for the lack of bloom.
Lilacs bloom on wood that grew the year before, so pruning at the wrong time can remove flower buds. The only time to prune these is in late spring or early summer, right after their normal bloom period.
Sometimes lilacs will fail to bloom due to having too much or too little nitrogen fertilizer. If your plant is growing in or next to a lawn that is regularly fertilized, that should be sufficient. If your lilac is planted in a shrub bed, you might try giving it an annual top dressing of compost along with an application of general purpose granular fertilizer such as 10-10-10 per the label directions in the spring. That, along with a routine mulching with three inches of organic mulch year round should be adequate.
If none of these sound like it might be the cause, you might want to consult with your local professionally trained nurseryman or with your local county extension.
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