There can be several reasons, Minnie. too much nitrogen or over-pruning are two of them. Here's a link with more information:
Most hydrangeas bloom on old wood. Which means if you prune them too far down you are cutting off further blooms. What I do on my (over 20yr old plant) is wait til I see the earliest buds starting in spring and only prune off the tips of the branches above those buds. It’s easy to tell then what is dead winter damage. This way I am not accidentally removing this springs blooms which the plant started making last year. To control size you can remove some of the weakest looking branches at the base for thinning. You CAN cut the entire plant down to 6” to rejuvenate it but you will sacrifice that summers blooms.
add some coffee grains :) and don't cut your bushed down in the fall or spring