Answer: If you are talking about traditional lilac bushes (genus Syringa), you can count yourself lucky if they bloom for you in Las Vegas. They are not well-adapted to southwestern desert growing conditions. Lilacs not blooming is a very common problem and a frequently asked question. First, very few lilacs will bloom before their 3rd season. The most common reason that older lilacs don't bloom is a lack of sunlight; they need full sun to only partial shade (at least 6 hours of full sun) to flower well. Without sun, the plant puts on plenty of foliage but no blossoms. However, in the desert's harsh sun, it's best if they receive morning sun and protection from hot afternoon sun.
Are you "feeding" your lilac? Add an inch or so layer of organic matter such as compost around the shrub each year. You can also add a phosphorus source such as bone meal, or a fertilizer with the middle number high (5-10-5) in phosophorus, which promotes bloom. Make sure nitrogen isn't high, which will promote foliage at the expense of bloom.
Most lilac species will bloom on "old" wood formed the previous year, so do any pruning after flowering ends or you will remove the bloom potential. Good luck!
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