Answer: Arborvitae usually transplant well, and your care should have been adequate since one is doing well -- assuming they are in similar locations. However, sometimes there can be an underlying reason why a plant is not rooting and doing well. Sometimes there is a difference in lighting (overhanging trree causes shade for example) or surface runoff causing a spot to be drier or wetter than average. Sometimes an encroaching tree root outcompetes the new plant. Sometimes there is mechanical damage such as from a weedwhacker or accidental breakage, sometimes there is accidental herbicide contact. If none of those things seem to apply, and if you have been careful to keep the root zone and surrounding soil evenly moist (not soggy wet)in the months since planting then I would consider digging it up to examine the roots to make sure they are reaching out into the surrounding soil. Sometimes encircling roots will strangle the plant or will stay within the original potting soil and suffer stress as a result. If this is the case you can try to untangle the roots and direct some of them outward to encourage them to grow. Then keep the soil evenly moist by providing a deep soaking as needed (avoid daily light sprinklings) and apply several inches of organic mulch over the root zone to help maintain the soil moisture. Keep watering as needed (check the soil several inches down with your fingers) until the ground freezes. Sometimes too the transplant shock is severe and a shrub will just fail to grow for no apparent reason. You might also want to discuss this with the nursery where you purchased the plant to see if they have any suggestions. I hope this helps you trouble shoot.
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