Yellowing Leaves on Mountain Laurel / Cause ?

I got one response to this, indicating it could be damage from voles. Upon inspection, I find no evidence of this.
Any other ideas please ??
Thanks !!
------------------------- Two years ago, I planted 3 mountan laurel bushes, about 8 feet apart. All three get the same watering, fertilizing, and sunlight.
The middle bush has leaves that are yellowing. Perhaps a 1/4 of the leaves are yellowing. The other two look great !!
Any ideas why this is happening ?
Thanks for any help !
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