The mistake that most of us make with plants that grow from canes is pruning them too early or late, and then not pruning them with enough vigor.
February is the best time to prune plants such as the butterfly bush, and you need to take the plants almost all the way to the ground. I used this method with all our butterfly bushes last year. They all turned out full and healthy.
Photo on left is bush before I pruned it yesterday, and at right is the stub of the plant.