Magnesium deficiency?

Plants are individuals and what works for one plant may not work for another, even if they're the same strain bought together from the same seed bank. She's showing starting symptoms of calcium deficiency - I would recommend two things: lower your PH to 6.2-6.3 (6.8 is well within the range they say you can grow weed in but truth-be-told, some micronutrients are much fussier than they tell you about in "how to grow weed" blogs or articles). I would add some calmag to her feed - but only once a week... and only if she continues to give you symptoms like this. The damaged leaves will not get better so don't expect that.... you want to look at the new growth to see if the problem has been fixed... they should be clear of any spotting/yellowing. Good luck!