Why doesn't my Lilac bush bloom?

How much sunlight is it getting?

Try this article, it helped me. https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-bush-not-blooming.htm

My father told me I need TWO lilac plants to get cross-polination. So I did get another, and it worked!

Perhaps someone has placed some lawn goods too close to it. I would dig a area around it, and make sure to put some GOOD SOIL in that area, and be sure the bushes, are free from weeds or other growth. You may want to feel the base of the bush, and see if it is free, and nothing is holding it back from good roots. Be sure to use good soil, and water occasionally, depending on how the rain is in your area, but do water in the morning, not at night., or in the heat of the day. This hot temp with water, will harm the bush, vs helping with water. We have 4 bushes, and they do come in diff. colors, which is lovely. Best wishes, J.

Try pruning, lop off the top.

Put fireplace ashes around the base

My daughter (southern Ontario) had the same problem and decided to cut her lilac tree back to about 4' in height. Two Springs later, it bloomed like crazy and is still continuing to do so. Check that the tree/shrub has good air circulation because if it's been relocated to a crowded spot (between other shrubs), mildew can form on the leaves and blooms will be less plentiful. I speak from experience on that!

Also ... again from experience - do you snip off the old blooms (if you're lucky to get any)? I never did that but one year, decided to snip off the finished dead blooms and I cut too far back on the stem thus, snipping off the next year's blooms!

Google "how to prune dead Lilac blooms" and get directions - that is, once you start seeing blooms again!! I wish you luck!