Certified GKH Gardening Expert
You may have purchased them past their spring bloom period. Since they bloom on old wood (buds produced the season before), you shouldn't prune till next spring after they bloom.
Soil fertility and water management and pruning could be the keys. Have you fertilized this summer? Watered adequately? With nutrient and water needs met, pruning next spring should make a difference for new, more dense growth next season.