Butterfly bush growth
Butterfly bushes should be planted in full sun. If your is in a too-shady location, it may be weak and/or reaching for sunlight. Plants over-fertilized with nitrogen also have weak stems sometimes. You do not need to fertilize at all.
If this is a new plant, it's possible that this growth habit may be somewhat characteristic of the variety your purchased. Provide enough sunlight and watch its development for a season. You may have to stake it. This is not the usual growth habit of butterfly bushes. They are normally very vertical. If its getting full sun and continues to flop, return it to point of purchase.