Overwatered, underwatered, too hot, too cold, too much light, too little light, dead roots... there are a lot of possibilities here.
Overwatered I think I most likely but I believe your light is too close also. Your model pushes 730 micromoles at 20". I don't think you want to break 1000 micromoles in flower (900 is often the highest recommended). Assuming your light is working properly, you should probably go up to 18".