Answer: Even though everything seems equal, the one that's not doing well is signalling its unhappiness by wilting and looking bad. I think it's worth the effort to dig it up and check the root system and the drainage. Sometimes soil in containers can develop air pockets in the soil; even though you think you are watering thoroughly, the water may be running out before wetting the soil around the roots. It's worth a check.
Best wishes with your verbenas!
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