I am growing several varieties of impatiens from seed. I have excellent germination under lights on heating pads. Germination is not the problem. Some of the seedlings are loosing their first leaves with none to follow. They look as if burnt away. The little leaves turn brown at the edges and curl up and wither away. I do not believe it is damp off as they are still standing straight up. After the majority sprouted, I shut off the bottom heat so I don't think they are too warm. They are about 4 inches from flourescent lights. Others in the same tray are thriving and about to be transplanted. Any ideas?