What's your EC? Sounds like a nutrient deficiency, not a timing problem. You might try increasing your nutrient strength by 10-25% to see if this corrects it.

Otherwise it might be a magnesium deficiency..but could also be manganese or iron.

Could be tough to say exactly. Most often, Magnesium deficiency results in greener veins with yellower leaves between them.

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Iron could be the problem. It often results in veins AND leaves turning yellow. Of course, if your PH is too high, your iron will become less and less available as your PH rises, until around 8 it becomes almost completely unavailable. Might want to check your pH.

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Manganese will look more like mottling(small spots) starting near the stem and working out.. like this:

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