
Certified GKH Gardening Expert
This could be due to overwatering and disease, or alkaline soils. I would check to make sure that the soil is on the acidic side, which they prefer.
Here are some articles that will help:
https://www.gardeningknowhow.com/ornamental/shrubs/cleyera/japanese-cleyera-information.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/test-soil-for-pests-and-diseases.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/making-sure-soil-drains-well.htm