When we got our lilac bush four years ago it was only 2-3' tall. It did great for a few years, and has doubled in height. Last year it didn't really bloom, expect for a few tiny things, but put on a lot of foliage. This year, part of it is blooming, one branch has underdeveloped blooms, and two major branches (making up about half the plant) seem like they might be dead despite large buds. What is wrong with my beloved bush?!? How can I save it?