If your growing inside, I'd bet that the lower portions of the plants aren't receiving enough light. Especially if you are pretty sure the amount of water and nutes is good.
Remember: The type/spectrum of light is FAR less important than the AMOUNT of light the girls get. Lumens is 95%, spectrum is 5%. During flowering, this may change to 90% and 10%. Still not too important!
If outside, and your sure there's no root or stem rot and no pests, I'd guess an acidic soil or lack of N or Fe (Fe not too likely). Acidic soils will lock up some of the nutes in the soil even if they are plentiful.