Q.Buddleia
Last year I planted a Buddleia in a large pot in a catch-all generic compost. It seemed to do well the first year and did flower. I also gave it a couple of feeds of potash. Seeing as this was the first year I didn’t prune it un March. Unfortunately this year it isn’t doing as well. It is not troubled by pests or anything of that nature however the leaves are turning brown and falling off. There is still some growth there but I was wondering if there is any way of saving it?

Certified GKH Gardening Expert
I'm wondering if it's the container. It may not be draining well or is being overwatered. Can you remove it from the container and plant it in the ground? Since you didn't cut it back in spring before new growth started, cut it back now by at least half. They benefit from being cut to about 8 inches in spring because they bloom on new growth. If you don't cut it back, all the blooms will be at the top. Spider mites can be a problem with butterfly bush. Spray the leaves with Neem oil weekly for a few weeks and see if that helps.
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-container-care.htm
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/brown-leaf-spots-on-buddleia.htm
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/how-to-revive-butterfly-bush.htm