I'll bet it's just a PH issue making it seem like the beginning of a Calcium deficiency. Just flush with an extra half gallon of water and your normal nutrient solution, but be doubly sure that your PH is around 6.2..maybe recalibrate your PH pen..
It coud also be that you're having root problems on that plant. They will manifest themselves as a deficiency, and if you know you've been feeding them properly and that the PH is absolutely correct, that's probably the problem.
You can try aerating the pot by using a skewer or somethhing thing and pointy to poke and prod the soil to loosen it up.
You may need to just transplant into a pot with more coco or perlite..some plants just need an airier soil.
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