Cant help too much this is my first grow so not much exp. I cant check my tray atm because they're sleeping but i'll have a look tomorrow see if any of mine are.

Light kills the roots, I have only sneaked a peak out of the corners twice to check the nutrient is going in ok is there any chance light could have hit them?

I read a thread about roots dying it said put some liquid oxygen in there and take off the dead root matter, maybe the oxygen is stopping more developing for you but the dead roots are still there maybe infecting surrounding matter.