Answer: Based on your description it sounds like your tomatoes need more light and are stretching to get it. When you use a greenhouse you are subject to the vagaries of sunny and cloudy weather; if you use artificial or supplemental light then you can control that as needed to produce stocky transplants with 14 to 16 hours of good light.
Another possibility is that your seedlings are damping off a bit which is causing them to collapse. This happens when they are too crowded and need better air circulation as well as more light. Overwatering can contribute to the problem, as well.
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