Answer: If slugs are the culprit you should see their slimy trails or rather the dried, shiny trails. Also check for some half eaten leaves, another evidence of slug damage. Since you cannot catch any in your traps, slugs may not be the culprit.
Impatiens often lose their leaves when stressed. Typical stresses include mite infestation, drought spells, low nitrogen, and root damage from soil insects. Try to keep them moist but not soggy. This can be difficult in sunny spots as the heat of the southern sun can demand more than the roots can supply. Carefully dig up a plant or two and examine the roots for damage or signs of swollen areas.
If you find any additional symptoms such as mentioned above submit another question with the details and we'll try to narrow the diagnosis. In the mean time, keep them moist and fertilize weekly with a liquid plant food. I suspect they will pull out of in in a week or two.
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