How many tiimes a day are you watering them? Is the RW mostly drying out between waterings? That's the ONLY other thing I can think of besides nutes or ph.....soggy roots not getting enough oxygen.

Hope you figure it out.

Good luck.
Weedhound Reviewed by Weedhound on . Can't figure out what's wrong... 3 weeks from bagseed, 1" rockwool cubes (pH buffered) and hydroton, 8" net pots in five gallon buckets with bottoms cut out. Flood & Drain / Ebb & Flow, 40 gallon reservoir. (res temp 71 F) RH = 40% Growroom temp = 75-78 F 250watt HPS light, with two 48" Floro tubes as side lighting, and a CFL for good measure. Rating: 5