Im using FF nutes, water with PH 6.5. been feeding 1/2 strength veg nutes along with flower nutes but i'm still having lower leaves turning yellow and eventually dying (N def?). I've lost lots of fan leaves and im only 3 weeks flower
I'd agree with it not being root bound. You'd notice that you have to water twice as much before it's root bound.
You may have nutrient lock out. Try flushing for a few days. Then start back up slowly with the nutes.