Answer: It does sound odd that only one plant is affected but even though they are planted in close proximity, the growing conditions may not be equal. Sometimes soil doesn't drain well in a particular spot and sometimes root damage has occured with one plant and not the other. If it looks otherwise healthy, aside from the slow growth, it may just be trying to adjust to the growing conditions. If it simply doesn't look healthy, I'd dig it up and check the root system and the soil drainage. You may find that a critter has damaged the roots or that the site is soggy or overly dry. If everything looks okay, replant the burning bush and water it well. Then prune it back a little to encourage healthy new growth.
Best wishes with your landscape!
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