Hi Happychicken, if these are young plants, my first guess would be they are being overwatered. Maybe the plants have not been kept in full sun and that was too much for them, plus more water than the plants can use, could cause the roots to rot. Might be better in diffused light, and pulled out of the pot to get air to the roots. Those little plants are much more fragile than more well established plants with strong roots. I hope they pull through for you.
Mike in Hanover, VA