Sorry you are having trouble. May I suggest you put a scrog net or some supports for your plants. They look stetchy and without support the plants are stressing. Seems like you have everything else right although you didn't mention ph, what do your roots look like and is there any algae? Have you adding anything new recently?! How long have they been without nutes? Is the res oxygenated? In hydro the plants are depending on you to supply every need, so they can't go long without food. Good Luck, and I hope you solve the prob.