Could it be Impatiens Necrotic Spot Virus ?
Small black spots which appear on the leaves are usually caused by thrips.. did you see any ? Also make sure the plants are adequately spaced so foliage can dry during the day. Avoid watering in late afternoon/early evening. Instead water after dew fall, or in the morning hours. (unless you can water without wetting foliage).

If you already have netrotic spotted virus, it may be very difficult to eradicate unless you are preparred to do some very thorough cleaning out.

One way to know, check the new growth. If the young leaves appear stunted (small, wont grow), this could be your problem.