Flowering - nutrient deficency?

Without a doubt, there’s a nutrient blockage, and something is definitely going on with the pH that is causing the blockage. Right now, there’s no need to place major blame on nitrogen contributions. You’re already in flowering, and any amount of nitrogen in the nutrients is no longer the most important thing, but what I think is that you’re definitely dealing with the wrong pH. What’s the Mg contribution you’re giving, and have you started with the flowering boosters? My opinion is that throughout this week, you should give some waterings with kelp (Ascophyllum nodosum), for example, the liquid one from Cultivers on Amazon is great, and first check if your pH is coming out lower than necessary or too high. At this point, my idea is that rather than focusing too much on the EC of the system, you should see if you’re watering enough; for me, during this period, you should always keep the pH between 6.2 and 6.6, due to the integration of what is now more necessary: magnesium, phosphorus, and potassium. But it could also be that you’re experiencing some water stress and most likely have low water in the system. Increase the water to 1 liter per day and keep the pH between 6.2 and 6.6, and if you balance the pH, you definitely won’t have nutrient blockages. From what I see, there are three essential problems at this moment - first, a magnesium blockage that is very clear, and when the magnesium pump isn’t working, phosphorus and potassium also have lower systemic availability. Kelp is incredible for optimizing nutrient availability. Then I also see that there’s a water deficit in the system. Kelp will almost immediately replenish nutrients that aren’t being integrated correctly. Watering daily with 1 liter of water between 6.2 and 6.6 meets the integration margins for magnesium, phosphorus, and potassium, and with that, you’ll recover the health of the grow in no time. A hug, and don’t panic, but you need to act very quickly... the pH is the foundation of everything that’s happening.

If the yellowing didn't start from the bottom going up it is more potassium related due to high demand in flowering. If it did start low then it is nitrogen related. Flowering npk ratio is 1-2-3.

Increase your micro, add a P/K booster and do what grey wolf said.

yes its nitrogen contrary to some peoples opinions the cannabis plant still requires nitrogen during flowering just in smaller amounts I would suggest a vegetative feed at half strength to rectify the deficiency however those yellowed leaves will not change back to green