I'm thinking about overwatering-stress but we're missing some informations here. What's your feeding schedule, have you already started to give nutrients? How often do you feed them with nutrients, and how often do you give them plain PHd water? Do you know the temperature and humidity level inside your domes? Also Jiffy pods are known to cause this kind of trouble, mainly because of roots rot. You may want to transplant them into their final container, then let the soil dry for a couple of days. I'll be around, good luck with these babies!