This is especially a problem in wet summers. Use a good fertilizer or Tree & Shrub with a high middle number. This middle number is phosphorus. Use dry fertilizer but not ‘garden fertilizer’ and use lime for Lilacs.
Prune out ‘old’ wood to open up the shrub and spray with fungicide. Benomyl is the best. Pruning takes place in early spring. Remove spent blooms immediately after blooming.