A lilac may not flower while it is establishing a roots system. Once that is done it can expend the extra energy on flowering. A lack of sun and phosphorus can slow down this process. Feed your lilac each spring with Flower-Tone by pounding holes out away from the trunk and pour the Flower-tone into those holes. Do this in several place around the lilac so more roots can find and use the food. If you have been pruning your lilac in summer you have been cutting off the buds for the next season’s flowers. Lilacs form the buds for next spring’s flowers this summer. Always prune your lilac in spring right after flowering so the buds form on new growth. If it is planted in shade then it may never flower.