I am sorry I can't answer you question because I live in a very different climate from N.J. I would recommend going to a nursery to ask their advice about your lilacs. I know in Northern California lilacs need to be kept watered until they are well established and after that they are somewhat drought resistant. Are your lilacs all planted in the same place? Try moving the ones that aren't happy to another location. Also know that even professionals have plants that fail. Just keep planting!