Very odd. Some more information about it's situation would help, but here's my first thoughts:
Lilacs need bright, full sun areas. It wouldn't thrive if it isn't getting enough sun.
Lilacs hate to have their "feet wet." That means it needs well draining soil that doesn't stay wet. And it doesn't want to be watered too much.
Lilacs don't like acid soil. You can have your soil tested. If acid, add some garden lime to increase the pH.
Is there a black walnut tree nearby? Lilacs are sensitive to being stunted by the juglone the walnut tree emits.
If you don't think any of these are it, tell us some more about what the conditions are where it is planted. Look at the leaves, is there any sign of insects or disease? Check the bottom of the stems for borer holes or signs of being chewed by mice or voles. Let us know what you find and we will keep trying.