Slug bait may not always work, but if there are slugs, Sheri's beer trap will absolutely attract slugs and prove they're there. I've noticed that impatiens are often attacked by pill bugs, rolly-polies. Sometimes they'll grow in spite of the damage and do fine. Pill bugs seem to be attracted to areas mulched with certain things. One fairly no-fail treatment for slugs and pillbugs is Diatomaceous Earth, it's not hard to find, especially at a farm and garden type store. Trim off all dead parts on the periwinkles to reduce stress and use a liquid fertilizer like Miracle Grow or Fish Emulsion to boost growth and overall health, then spread the D.E. when the soil dries.