A common question.
And the quick answer – because they were pruned too late in the season last year.
The new buds that produce those glorious spring displays start to form shortly after bloom time is complete in the spring. So if you start pruning mid-summer or fall, next spring’s bounty will be cut away.
Clip off the spent flower bundles as soon as they are finished early spring and do any additional pruning the plant requires at that time. Cut back any dead, unhealthy wood and remove any suckers or sprouts at ground level.
Then sit back and wait for next spring’s show of colour!