Answer: To see if you need to water, dig into the soil with your finger. If it is still damp, you do not need to water yet. With a newly planted shrub such as this, it is a good idea to check the original potting mix close to the stem as well as the surrounding soil as they may dry at different speeds.
Since one plant seems fine, it may be a case of this plant having been rootbound or some other root problem. If the roots were encircling the soilball, or if the roots were excessively dense, it may not be able to take up enough moisture from the surrounding soil. The way to handle that (since you just planted it) would be to dig it up, loosen or cut the roots and direct them outward. You might also trim back the branches to compensate for the root damage.
You may also want to consult with your retailer since it is so new.
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