That could be several things, how far is your light above your plant and what does the manufacturer recommend for height above a seedling? It could be light burn but I dont know if light burn will stall the roots or not. I usually usually top feed a little every day until I see swarm of roots coming out of the net pot. What are you feeding the roots to encourage them to grow? Are you using Hydroguard? What is the water temperature ? 

Have you been to Growweedeasy.com?   

https://www.growweedeasy.com/5-step-solution-to-most-cannabis-growing-problems