Thanks for writing. One possible cause of the kind of situation you describe is that the ailing plant has an air pocket in the hole where you planted it; in this case the roots tend to dry out quicker. Sticking the hose in the ground at various places in the planting hole can collapse the pocket.
Were the roots damaged when you planted it? That can also cause one plant to suffer when others that were planted at the same time are thriving.
Does it get as much moisture as the others? Sometimes because of sprinkler patterns, it turns out plants don't get evenly watered.
---Justin, Senior Garden Editor, BHG.com "