Just a thought, you may want to put more row rocks into your cups, around the base of your plants. Fill those cups up as much as you can because humidity is seeping up through those net cups. Maybe your lower leaves are yellowing because they are close to the humidity near the grow rocks. Also, maybe its time to start giving regular doses of nutes, maybe theyre about to go vegative stage and are now looking for more nutes. Just my guesses. And if you have those long fluorescent lights i would suggest keeping them very close, like an inch or 2 away from the canopies.