Certified GKH Gardening Expert
This could be a number of things--too little or too much water, over fertilizing, leaf scorch (if planted in too much sun), etc. It may help to prune the plant to help rejuvenate it. Here is an article you may find helpful: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/prune-hydrangea-bushes-hydrangea-pruning-instructions.htm
In general, you want a fertilizer that is higher in phosphorus. This helps with blooming.