Mature lilacs respond well to pruning. Not just pruning, but the removal at the base of bush of older, large limbs. It's not too late this year to go in and remove most older, large branches from base of shrub.

You can also remove some other, not so old branches too. Keep in mind that lilacs bloom from last years growth. So don't remove too much of the younger branches. But will not hurt it any to clean up the inside of the bush. This will force it to put on new branches and bloom well next spring.

I have 3 mature lilacs on property. I usually prune the "hard" bout every 3 years.

Here's my 20 something lilac, not the most impressive amount of flowering! But, at least, there are some blooms. When I prune, I usually take my electric hedge trimmers to it in late June. But this year, I think I'm going to try to trim with my pruning shears and loppers....trim off 8-12" back to the start of the newer growth. Same with the nearby yews, I like their free form look, but I don't want them covering up the windows. Also, here's a picture of my viburnum, it's bloomed better than ever...go figure. I'm in central IL, so we get lots of wind and below zero temps AND I'm not big on watering things.

Thanks everyone. I really wanted to know if it was ok to prune the bush back , even though it did not bloom this spring. (And my dwarf bushes are blooming fine - it's the large one that didn't bloom this year.)