Any source of iron in the nutes or soil? Any micronutrients? Are you spraying them too much foliar feed?

What is the ph of the water? Might be a ph problem causing nutrient lock-out. As my well water is usually around 7.9 I use a cheap and easy-to use freshwater aquarium ph test kit to make sure I've adjusted it right. ($7.00 u.s. or so) Perhaps a 1 tablespoon/gallon Epsom Salt treatment?

I've never used it, but does Fox Farm soil already have nutes in it? Doesn't really look like nute burn...

I'd wait for more answers before jumping into a solution, tho.