There are various reasons for yellowing leaves on a Forest Flame.
This plant likes a acid soil so it can be fed with a lime hating fertilizer.
If the soil is high in alkali it will diminish the amount of chlorophyll in the plant resulting in pale yellowish leaves.
Pieris can also be vulnerable to the Lace wing insect that will feed on the leaves and leave them mottled in appearance.
Use a horticulture oil or insecticidal soap to treat for Lace Wing. Also using a strong water spray to remove the eggs from the leaves can help.
Other reasons for yellowing leaves on any plants are watering issues, to much or to little.