could be a lot of things. nute burn would be my first guess. whats your ppm? soil or hydro? ph lockout would be my second as its common. what do you ph your nutes to? could be deffiecient also (maybe from ph lockout). what does your new growth look like? if its thin, small, contorted, or twisty then defficiency sounds like the answer. if your sure they're not defficient then they could have toxic salt buildup (lockouts nutes). if vertical growth has stopped then you know its defficient. i would say flush with water with 3 times the amount of medium and hit it with a 3/4 strength nute dose. what line of nutes are you using? hard water, ppm?