It's hard to say why your violas are not hardy, but they can be picky about hot and cold weather extremes. Here in Iowa, for example, they won't come back for me either. I do get Johnny-jump-ups all over the yard, but the hybrid violas do not really like certain parts of the midwest because of the temperature extremes. Violas are also not long-lived perennials so I would plant them, enjoy their blooms, but not expect them to be a constant companion.
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