That often happens with group 2 clems if they don't get pruned.
Take off all the dead heads as soon as they die. Do not let them waste energy on seeds. Give them a boost with a generous drink of liquid rose or tomato food and a top dressing of slow release blood, fish and bone or pelleted chicken manure. They should then produce a new flush of flowers in late summer and should also produce new growth lower down for next year's spring show.
You can also cut one main stem per year at the base and, when it's all wilted after a few days, pull it out gently. This weill help generate growth form the roots and new stems will flower lower down. Next spring, feed as above in early spring as soon as new growth shows.