Are you watering them according to some schedule, or whenever the soil is dry? The soil does look a bit muddy, which could deplete the roots from oxygen (and plants this young are particularly prone to this kind of problem) and result in several different problems, including yellowing leaves.
I wouldn't water them unless they are significantly lighter (pick them up after watering and you'll know how heavy they get vs. how light they are when dry), which could take anything from a day to almost a week depending on your climate. Keep trying to save the worst-off one, my biggest yield so far was from a plant that went through an even worse time than yours.