i want to make it clear before i try and help that i am in no way an expert grower, i just want to try an suggest some things and maybe the experts will pop in and know what exactly to do..

one thing that i first thought looking at that picture is that it possibly could be rootbound, how big is that pot your using? it really doesnt look big enough for a plant that big..

another thing could possibly be overwatering, or over nute.. thats all of my knowledge (which isnt a lot)

but like i said i just want to help but i cant tell you from first hand experience, so wait untill you get more responses.. other than that just let the soil dry out and flush it again, maybe even try rasing the light up just a tad to see if that has any effect? good luck man, hope this helps