Well, I don't have any answers for you. However, I'm having the EXACT same problem. I have a small bed to the side of my step. There mixed impatiens planted in front of my plumbago. The impatiens on the left side have just died with no logical explanation that I can think of. The ones on the right side are doing perfectly. I don't understand. I too look forward to an answer.

Is the soil different where they are dying. I know in my yard I have some spots that are worse then others. Also could there be something under the ground that is drying their roots up, like a large rock or the concrete from the posts.

I ask about a rock because I had a similar problem and when I finally got fed up and went really digging I found a brick. Who would have "planted" a brick I dont know but until I found it and removed it nothing would grow on top. It was about 8 inches down.

Good luck

carey