I have two butterfly bushes in my yard in NE Ohio. Each one is planted in two opposite corners in my backyard, planted in sandy soil. Both are watered regularly with the sprinkler system and both are along the west side of the yard. The butterfly bush planted closer to the north is THRIVING. The one in the south corner had a few flowers and the leaves are falling off the stems. I cut back both bushes in February or March of this year.
Certified GKH Gardening Expert
I would suspect that a fungus is attacking the roots or leaves of the sickly plant. Treat the plant's leaves with a fungicide and mix some fungicide into water and water the plant with it. I will say that if it is a fungus on the roots, they tend to be more resistant to fungicides and may not fix the issue. In which case, it may be best to remove the plant before the fungus spreads.