It sounds like your lilacs may be suffering from a disease, possibly lilac bacterial blight. This condition can cause the leaves to turn brown, and it's often characterized by dark streaks on the shoots and wilting flowers. To combat this issue, you should prune any infected branches about 10-12 inches below the visible infection as soon as you notice signs. Clean your pruning tools between cuts to prevent spreading the disease further, and ideally, destroy the pruned branches by burning them.

Moreover, it is recommended to avoid over-fertilizing your lilacs, particularly with nitrogen, as this can encourage disease development. Instead, make sure to fertilize appropriately and stop around mid-summer.

Since you've mentioned you have a mix of healthy and brown lilacs, it’s a good idea to increase air circulation and conditions that can reduce the spread of disease. Consider removing dead wood and ensuring that your plants are not too crowded together,

Attached are some sources that provide further information:

https://extension.oregonstate.edu/news/how-recognize-treat-avoid-lilac-bacterial-blight

https://extension.umn.edu/plant-diseases/lilac-issues-and-diseases

https://extension.psu.edu/lilac-diseases

I trust this helps. If you need further guidance, please send photographs showing the problem