There may be nothing wrong with them. Make sure you do not water until they are very light to pick up. Let them dry all the way between watering and get them as much light as possible.
get it on 18/6 i wud recommend, plants grow alot wen in dark hours, dnt think that is ur prob tho, looks like it could be over watering.
plus it will knock ur energy bill dwn alot.
like mass says dry b4 water.
gl