I cannot offer any firsthand experience with this, but did find this when researching......................

http://treesandshrubs.about.com/od/commonshrubs/p/Common-Lilac-Syringa-Vulgaris.htm

It lists some of the things that may be wrong with yours.

I had a good friend in Galveston years ago that said, when I mentioned I wanted to try a lilac shrub down there, "Peggy, don't bother. I have tried some 5-6 times and I just can't get them to grow down here." And this guy was famous for being one of the best gardeners in the area. Nothing he did kept his trials alive and he was convinced it was the excessive heat we got there, as well as drought that got them. We had a huge one in Montgomery Alabama years ago along a driveway. It sot sun on one face and shade on the other, so I know they will do well in the South in the right spot. But maybe not in Texas' brutal summers of late. Best of luck with yours coming back with TLC though.

I had pretty good luck with a couple of Persian Lilacs in Temple at my last house. They had been in the ground for 3 years, were about 6' tall, starting to branch and bloom nicely in the Spring the last year I lived at that house. They demanded a lot of water as they were in full sun along the outer cedar fence in two edged beds. But they were shaping up nicely when we sold the place last February. I had told the realtor what they were in hopes they might keep them. Had a lovely scent, but the bloom clusters were still fairly small at best, as I forgot to fertilize them with any regularity. The buyer has since cut both of them down and replaced potted plants in those two exterior beds. I was sad when I drove by recently and noticed the lilacs were gone. I just live 6 blocks and was so hoping to watch them grow and increase in beauty over the years when Ii pass by the place...........but that is not to be now, it would appear.

I looked up the climate for El Paso, and on average it does get very hot in July. Heat is a problem for lilacs. They also do not like humidity which is not a problem in El Paso. However, overhead watering from sprinklers can give rise to several diseases. Sometimes pruning and applying a fungicide/bactericide can save a plant. HERE is a list of common lilac diseases which might apply. I use a product called Physan 20 which is both a fungicide and bactericide. It treats plant diseases such as fire blight, anthracnose, black spot, and various rusts, rots and mildews.