What kind of soil are you using? Is it a sterilized medium? You may have a critter in your soil that comes out to dine and goes back in the soil to sleep during the day. You will not see him. Are you looking for mites and catapillars? Leaf loss for young seedlings is a serious problem. I suppose you have looked for the obvious culprits. Try some malithion. It leaves a white residue on your leaves and most pest fear it, with good reason.