Lilacs usually don't need pruning unless they are old and overgrown or if they have dead or crossing branches.
Lilacs set flowers in the previous years growth, any pruning should be done right after the flowers have faded. Pruning too late in the season will cut off next years blooms.
My advice: Make sure it gets enough light, Lilacs won't bloom unless they get full sun. Fertilize it with bone meal in the spring. Don't prune it, it will eventually fill out and get bushy if it is healthy.

Giantslug, thanks so much for the info! I will follow your advice.

Linda

I had a similar problem with lilacs in one location of our property....but I know they were not getting enough light. Eventually they did start blooming and I carefully prune them when they are done...if I can reach the blooms. :)

Another thing to consider, and I know you know your gardening...but is there any chance they got planted too deep? I know that with many flowered shrubs, if they are planted too deep they will not flower. I don't know if this is the case for lilacs, but I assume so.

Good luck!
Kristy

Kristy, I don't know the answer to that. I'll have to read up and see if I planted them too deep.

Thanks for another idea!

Linda

Just to confuse things :) Years ago I planted some lilacs and they just didn't do anything for perhaps 3 years. Then my neighbor boy (well-meaning) thought he would help me out my mowing over that brush. (!?!) He mowed them down, and they took off the next year like gangbusters--nice and thick, and in another year they were blooming. Perhaps they were just ready, but I don't think the major pruning really hurt, and it probably helped to bush them out.

We had a squirrel that helped with landscaping one year and planted a black walnut tree for us near a new Miss Canada lilac. Needless to say, it did poorly. The ones that had been there for years were fine. Do you by any chance have one of these offenders nearby? (Oh, we left the walnut because it was actually in a good place! LOL)