Is it getting enough sun?
Pruning in winter is not advisable, as you don't know what you're cutting. Try pruning in Spring after the buds have started growing.
Try fertilizing c Bloom Builder in Spring.

I would think the problem is not enough sun. It could also be a nutrient imbalance. Too much nitrogen relative to phosphorus and potassium can cause lots of green growth without much flowering.

Pruning should not be an issue since they bloom on new wood. I cut mine down to about a foot tall in the spring. You can wait to prune until you see new growth to determine how far the die-back was in winter. Leaving it up in winter protects the plant somewhat and provides winter interest too.

For me the store-bought butterfly bushes always performed poorly. They usually only lasted a year or two and never got any size to them. Then I wintersowed some seed I got in a swap and have had the resulting plants for almost 5 years now. The one in full sun shoots up to almost 7' tall every summer after cutting back to 12" in spring. It is loaded with blooms that I deadhead to keep it blooming until a hard frost. I have never fertilized it, but we do have a more clay based soil that retains nutrients. I think some of the more cultivated varieties and hybrids have probably sacrificed hardiness and vigorousness for the sake of compact size and new colored blooms.