Answer: Check their soil moisture. Too wet or too dry can cause probems especially in a hanging basket where the roots are confined. Also, some soil mixes are so coarse and airy that they don't hold water well while others are so "mucky" that they easily become waterlogged.
Curling leaves can be in response to mite infestation. Look under leaves or tap a leaf over a white paper and watch for the tiny critters that look like reddish-brown specks moving around.
A long shot consideration would be herbicide injury from a broadleaf weed killer used on nearby turf. A treatment to the lawn can volatilize and drift, damaging sensitive plants in the area. Using a sprayer to spray your impatiens plants which had previously had a herbicide in it can also cause damage.
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