There are very few leaves that turn red. The bushes are about 7 years old.
Burning Bush Leaves Will Not Turn Red in Fall
Certified GKH Gardening Expert
This actually is quite a common complaint about Burning Bush.
I myself have on shrub that is a brilliant red color and one that has only a hint of color.
Proper plant sight with good sunlight is important.
Applying a light fertilizer in the early spring will help the plant.
Here is a link to refresh you on the care requirements to optimize the plants growth.