Here are several reasons why a common lilac (Syringa vulgaris) may not bloom. It was pruned between midsummer and winter, removing its flower buds for the next season. Prune only in late spring, right after normal bloom time. It's an old variety of lilac that takes 6 years or more to bloom. You may simply need to be patient! It needs full sun. If the plant receives less than 6 hours of direct sunlight daily, it will never bloom well. Your winters are too mild to allow it to set buds. Common lilac grows best in cold-winter areas (Zones 3-7). If lack of cold is the case, a Meyer lilac (S. meyeri) may be a better match for your garden. Meyer lilac blooms at a young age and it flowers better in the South than most other lilacs.