It looks like you are having a root issue. If the medium has been kept too wet then this can cause a lack of oxygen and allow root rot to set it. I would give them a dose of 2 tsp per gallon of 3% hydrogen peroxide and that should kill any bad bacteria that is killing the roots. Then let the pot dry out pretty well between waterings and make sure you have good drainage. Plants have a hard time recovering from major root damage, but hopefully you can get it turned around. Good luck!