Lack of sufficient sunlight and too much nitrogen in the soil are reasons a lilac blush does not flower. Another reason a lilac bush does not flower is an incorrect amount of pruning at the wrong time.
A lilac bush needs at least six hours of sunlight per day to bloom so it is best to plant it in a sunny spot. Also, lawn fertilizers put nitrogen into soil, and while this causes a lawn and the lilac bush's foliage to turn very green, it can prevent the bush from producing flowers.
To properly prune a lilac bush, remove dead or damaged wood from the bush in springtime and reshape the bush after the flowers bloom and fade. Always complete the pruning before midsummer to ensure the bush has sufficient blossoms for the coming summer.