Answer: Did you get any "true" leaves on your seedlings? That is, the first new leaves after the "seed" leaves. Or is the tip of the seedling missing? The first thing that comes to mind is that some critter--a mouse?--is nipping off the tips of your seedlings. They munch on the new leaves, leaving behind a bare stem. Since the growing tip has been chewed off, no new leaves can form. If possible, cover your newly planted seeds with one of those clear plastic domes, to keep those pesky mice away!
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