Gorilla Cookies struggling at day 65 – same setup, what’s wrong?

This is burning from a salt build up with a possible associated nutrient lock, usually caused by not getting run off each and every time you irrigate. John is correct, flush this plant and feed afterwards...........and it is not a magnesium issue or too much light. Damaged leaves will not recover, in fact they will probably still get worse over the next 10 days or so, before stabilizing. Remember too, what you are seeing in the leaves now, is a direct result of what was happening in the root zone 2 weeks ago..........had you asked this question when the sympyoms were first appearing........... Your other plants may look ok to you, but if they have been receiving the same treatment as this one, issues are only a matter of time, so flushing all of them may be necessary. It is a pity that you don't have a diary or did not post any pictures of your other plants for us to see. A pH of 6.3 is fine in soil.