What is happening?

Some great answers here already. Just want to add, it's also worth checking the PH of the solution going in and run off coming out when you see deficiencies. The plant uptakes nutrients in a certain pH range and will struggle to get what it needs if its ( pH) out of wack. Google search of pH and nutrient uptake for cannibas will show some graphs that explain it really well. I had similar issues with my first run and kept upping the nutes without effect. low solution pH was the underlying issue. Once that was in range she was off to the races. Don't stress dude you'll get it back on track.

Potassium, calcium and trace element deficiencies, all at the same time. Easily fixed with P/K booster, cal/mag supplement, (from grow shop) and a dose or three of soluble trace elements, (available from any garden centre).

Calcium deficiency the upper new leaves start fading in color and get the spots

Potassium and phosphorus deficiency