Lack of oxygen in the root zone. Make sure your water is about 20 degrees Celsius. 3% hydrogen peroxide in water mix will add oxygen to roots. Adding an air stone to feed air to roots constantly is probably the best idea. I hope this helps, good luck and happy growing
Hey! Happy growing my friend! I hope you do well and get some good results! I will come by sometime to see your progress!
If you ever visit my page please say hello!
Peace & love L.S.T