I wondered if I need Ericaceous compost. I have two other buddleias with no problems though.
Buddleia Bush Leaves Turning Yellow but Still Flowering
Certified GKH Gardening Expert
Yellowing leaves generally indicates a watering issue, to much or to little.
Insects also can become an issue that may cause yellowing of leaves when the plant is stressed.
Adjust water as needed and inspect the plant for signs of insects or disease.
Neem Oil is a good treatment.
Here are some links for you.