Burning Bush Foliage Yellow
We have planted burning bushes and they have turned yellow and somewhat thin. They are mulched in and fertilized annually. We have applied iron and still very yellow. They turn red very early in the fall.
Certified GKH Gardening Expert
I would have the soil tested. While you are fertilizing, if there is not the right combination of nutrient or not enough of any of the nutrients, it can cause this. In the meantime, it would not hurt to give the plant some potash for potassium.
Here is an article which will outline some other possibilities: