1) if your lower leaves aren't getting enough light tue plant will start focusing its energy on areas that are getting sufficient light. The leaves at the bottom cannot harness enough energy to properly synthesize food and the plant naturally cuts its losses.
2) air flow. If your not circulating air throughout the plan, carbon levels will become uneven and can create the same problems as a nute defeciency. Also the plants transportation is hindered without proper air flow.
3) switching to bloom nutes immediately after light switch. Your plant doesn't need bloom nutes for 2-3 weeks after flower starts. If you have gone directly into full bloom solution, your plant could be taking up too much potassium and phosphorus trying to meet its nitrogen needs. I always leave straight veg nutes first week of flower, and slowly increase bloom ratio 30% each week. Thus beginning full bloom regiment 3rd week of flower (or as soon as pre flower is over and bud sites have formed.)
If your using organic material in your solution, smell it. If it smells sour or fermented it could be old, have gotten too hot, or has contaminants. Any of these things will change the npk and the availability of materials in your media.
If your using rockwool cubes, try lowering your water enough that the rockwool isnt getting wet. Rockwool can do funny things to ph as well if it wasn't leached properly. Your ph of the water might be 5.8 but if your rockwool cube has been getting saturated this whole time, its ph could be drastically higher/lower and could even have excess salt build up. Try top feeding ph water as a flush next nute change and leave the water level about half way from bottom of res to bottom of net pot.
