I've just been through something similar with my babies. They had just reached 12 days old and started to turn yellow. They were in 100% coco, so I repotted them in soil. A few days later, they still seemed to have stopped growing and the lower leaves were still yellowing. So I fed them some Grow Big about 1/4 strength and suddenly new leaves popped out and they've started to grow again. Any new yellowing seems to have stopped. Next time they need watering, you might try a little bit of low strength food and see what happens. I think starting them in coco where they have absolutely NO nutrients, makes them use up their store of energy quickly. Next time I'm planting in soil from the get go. Good luck!