ok seems that ya might have a little root problem!
what else dya have in the medium rooted into rock wool! or what!
try moving the dripper slightly to the side, and hit the plant with some mineral magic, this will rid any root rot etc it may have, although if the same sollution is used it will go to the rest of the plants,
as you have the other plants looking good! it has to be at the root of this one!
sometimes when the flow of a dripper is too high, and too close to the plant it can have the effect of drowning the roots and killing them off!

good luck


welcome to the world of endless possibilities,,,,,hydroponics