Well sometimes butterfly bush can get pretty scrappy looking over time. Also, they need to be in full sun to be fully leafed out and bloom well. You don't say how much sun yours get, but at least 6 to 8 hours a day is necessary. I would leave them alone now, but prune them back hard in the early spring just before they normally leaf out. Wait till you see new growth beginning and then cut them back. You can prune back to any height, but going back as low as 12 inches is fine. They grow very quickly. At that time, I would also sprinkle some slow-release granular fertilizer around the base of your plants. A product such as Osmocote should work well. You can buy Osmocote at most garden and home stores."