It could be the location. Japanese tree lilacs are a full sun tree. These seem to be in a small area, with patio blocks over their roots and limited sunlight. They also need a sufficient amount of water and I do not know how much is available there because of patio blocks will cause most of it to run off. The tree is leafing out only the amount it can support under those conditions. You can cut off the dead sections and hope but I would consider transplanting them.