Answer: You have asked an interesting question. Lilacs are usually verty reliable bloomers. Sometimes the buds can be damaged by winter winds and thus not open correctly, this would be more likely if the plant is in a very windy location. Sometimes frost can damage the buds, this would be more likely if the plant is in a spot where heat reflects off a building for instance and encourages it to come out of dormancy faster than normal in the spring. It would also be more likely with a very early blooming variety. There might also be a disease problem such as a fungal infection interfering with the flowers. You might want to consult with your county extension if it happens again and see if they can help you diagnose the cause more specifically.
Q&A Library Searching Tips