I wasn't paying attention to your pics and see that you're doing hydro, not soil. I've only grown in soil so I can only speculate. Have you checked your pH, it could be causing lockout?
well first of all, your roots should be white. not yellowish brown like that. second, check your PH, and that looks like your are getting locked out of a couple of nutes. so PH is my guess. i hydro you want a PH of about 5.8
what is the temp of your nute solution? do you have an airstone in the bucket? how often do you change your nute solution? are you using tap water or reverse osmosis? how long is photo period? what kind of light? whats your pH? whats your PPM? is whatever is holding the stem into the cup smashing the stalk crimping it? what are daytime and night time temps and RH? there are a ton of variables...you could be giving it to much of one nute and be blocking out another...