If your plants are very old, they may be types that naturally lack large or long-lasting flowers. Here are some ideas for reinvigorating your lilacs and increasing their blooms in the future. Lilacs (Syringa) prefer slightly alkaline soil. If necessary, amend the soil with lime to make it neutral or slightly alkaline instead of acidic. Feed your lilacs with some 10-10-10 fertilizer if they've been growing in the same spot for a long time.
Make certain your lilacs are getting plenty of sunshine. They bloom best in full sun. If surrounding plants are shading them, prune excess growth to let in more light. If the plants are tall, old, and scraggly, prune back one-third of the oldest, thickest stems to the ground. Follow this routine for 2 more years until you have renewed the entire shrub. A more radical pruning method is to cut all stems back to about 6 inches in March to promote new growth. The next year, choose the healthiest stems to form the structure of the renewed shrubs and eliminate the rest. Prune the remaining stems to 1?4 inch above a pair of buds to stimulate the formation of side shoots. This means no flowers the next season, and perhaps for several years afterward if pruning is severe, but the plant will make more blooms in future years.