Thats aeroponics not rockwool.It's probably the light being too close or Nutrient deficiency.Possibly overflooding the roots.Maybe your watering too often...that causes root rot and will kill your plants.
Post a picture or noone can give you the right answer...
...right now everyone is just guessing and expiramenting with your plants is a very good way to kill them.So don't do anything till your sure what it is!!!