Sounds like an nitrogen deficiency.Fertilize with a nutrient high in nitrogen.Example ra-pid-gro 23-19-17,which means it has 23% nitrogen N,19% phophorus,& 17% potassium. N-P-K.The other thing is that i think ur ph level is to low i would say anything between 6.2-6.8 is just fine for marijuana to grow,and is the plant receving enough light is the top of the plant shading the lower leafs. anyway good luck.