Answer: This is a very common problem, one I have experienced myself. To be honest, I have seen very few lilacs that bloomed before their third season. However, here are a few bits of information and ideas to consider.
The most common reason that lilacs don't bloom is a lack of sunlight. They need full sun to flower well. Without sun, all you get are leaves! If the plant is shaded, you might consider moving it, or removing whatever is shading it, if that's possible.
Are you "feeding" your lilac? A good hefty addition of organic matter (compost, composted cow manure, leaf mould) would serve her well. You can also use a balanced, slow-release fertilizer.
If your lilac doesn't flower at all in the next couple of years, and you are really desperate, try root pruning the shrub. In late summer, dig one shovel's depth deep around the drip line of the plant. This will sever some roots and maybe shock the plant into setting flower buds. But if the shrub isn't blooming because it is weak or improperly planted, root pruning might weaken it further. Root pruning a young shrub really is a last resort. Good luck!
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