IF you're using the Greencare stuff (the original), it is designed to be used at about 3x the concentration you're using.
What you're talking about though, is not likely to be caused by a nutrient deficiency. The plants would just plain slow down if that was the issue. However, the pH could be affecting things. I suggest that you check the pH of the solution you're using to start with. (Assuming your medium is truly inert, it should not affect the pH in the least)
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