Shame about the earlier seedlings but great to see the new one's striking.
That photo of No3 on the 8th Feb show's an example of soil being much too wet. As Minnesnowta says damp not soaking. It takes much too long for soggy soil to dry out and stem rot could happen. With your new one's try to go much lighter with watering. Its hard to be patient I know, and many people think more is better, but never in growing seedlings. They will do well with minimum tweeking and over-loving
If you think there's the possibility of your soil carrying mould's etc, there have been threads here about sterilizing soil, but its not something I've needed to do (touch wood) so can't comment with experience.