Answer: It's always confusing when you have identical plants and one thrives while the other struggles to simply survive. There are no particular insect or disease problems associated with burning bush, so I'd suspect a root problem. Although the shrubs are planted close together, drainage and even soil type can vary from planting site to planting site. I'd dig up the ailing burning bush and inspect the root system. Look for galls, girdling (circling roots), rots, etc. Healthy roots are creamy white inside; rotted roots will be mushy, or brown or rusty colored. If you find root rot, the cause would be poorly draining soil. If you decide to replace the burning bush, do some work on the planting site to ensure better drainage.
Best wishes with your landscape!
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