Answer: Lilacs can get leggy when they aren't routinely pruned - and since all of the flowers develop at the ends of the branches, yours probably has only a few at the very top of the plant. I know it sounds mean, but cutting the branches back by a couple of feet will encourage new growth lower down on the plant. You may sacrifice flowering this year, but you'll end up with a shorter, fuller shrub. Do this for the next 2-3 years and you should end up with an attractive lilac bush.
Best wishes with your garden!
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