Where do you live ? Lilacs take time to get established and IMO are best where the winters are a deep freeze they seem to need that freeze to do well. I lved in Calgary most of my life and we had lilacs that were crazy full of flowers every spring.. I need to see yours to advise pruning but they can take pretty heavy pruning for sure. The big thing is to remove the dead flowers in the summer to allow the tree to get stronger.

What kind of lilac and when was it planted?

Common or French lilacs are suckering shrubs and will become thicker/bushier as they mature, producing new stems from the base. Pruning generally does not help with this feature.

I live in NY zone 5. Tree was planted 12 years ago. All of the lilacs I planted on this hill are struggling or died. I’ll post a picture soon

The ideal spot to plant lilacs is in an area with full sun (at least 6 to 8 hours per day)—give them too much shade and they may not bloom. Lilacs also like slightly alkaline, moist, well-drained soil. The best time to plant lilacs is in late fall before the ground freezes. So get the soil tested and of course they need the sun. I aslo agree you need to know what kinf of lilac you have so take a branch to the local garden center

Hill is in full sun all day. I’ve been adding aged horse manure around base of plant