It probably started around week 5
Your plants need nitrogen for the stretch. So the first 3 weeks of flower, I still feed my plants grow and bloom formulas and then at week 4 and 5 you have to give your plants calcium and magnesium at this time. Your plants are about to swell and build buds.
Calcium is the plant cell wall building block and if it doesn't have access to this at this time, it will steal it from the leaves.
Both of these things, the nitrogen and the cal mag at these times will help keep the plants happy and healthy from beginning to end of flowering.
You don't want nitrogen deficiency early in flower and you don't want leaf necrosis from cal mag deficiency at the end of flower.
Remember this for next time.
