are you pruning it in the spring? if yes then you are cutting the flower buds off. prune only down to the next bud. if the plant gets too big then remove 1/3 of the big stems, leaving the others to flower. then over 3 years it will be a smaller tidier bush.
in the spring give it a feed of fish blood and bone meal as a top dressing.

welcome to goy too

You should also leave the old heads on to protect the new growth from forst for the following year, then just dead head the old in spring not cut off. As the new growth come under the old.