Yellowing leaves in this shrub is most likely caused by too much sunlight.
They like to be in a more dappled sunlight area.
Other issues can be nutrients, alkaline soil and pests.
You can feed the plant a general all purposed fertilizer in the spring.
Check soil ph if you suspect the soil issue.
Inspect for insects and treat with Neem Oil if any our found.
It can take a long time to recover, even after the issue is resolved.