Why is my impatiens sick?

I bought two of these impatiens (double flower variety) about four months ago. I potted them both in the same potting mix and they are both in the same part of the garden (same shade, water etc). One is thriving, this one is very sick. The leaves are yellowing and curling, it never flowered, and the underside of every leaf has this spotted pattern. Is this a pest, or a fungus, or something else killing this plant?