Nutrional Lockout is a pH problem.
A simplified definition of NUTRIONAL LOCKOUT:
Can you imagine sitting a plate before a child at dinner time, with his most favorite food, hot dogs, ketchup and french fries? But also on the plate is a major portion of steamed broccoli, which he is just not fond of and insists on nibbling on. Now imagine telling that child, "you can not eat the hot dog and fries if you do not eat all the broccoli too". NUTRITONAL LOCKOUT is when the child responds with "well then, I just won't eat!"
Most nutes are nutritionally and PH balanced. But after being in your tank for 5 or 6 days, and being eaten from for 5 or 6 days, they become imbalanced. Perhaps you are growing plants that ate all the nitrogen first and just snacked around the iron, magnesium and calcium, or visa versa. The results are discoloration in the leaves, yellowing or rust spots, or curling up of leaf tips. It also becomes apparent when your plants were consuming a gallon or half gallon of water every day in your tank, and then suddenly when you check the levels the next day, they did not drink any water at all. This is NUTRITIONAL LOCKOUT.
You need to do a serious FLUSH, skip 12 hours of feeding and re-feed, less amounts.