What is happening??

6-7 is a wide variance... get that window smaller, as it could be th ecause -- 6.5-6.8. (if you are consistent, you can adjust ferts and keep that lower pH, it really is a matter of balance as opposed to one way or another). it's a mobile nutrient, so you can eliminate many possiblities with that info (bottom - up impact, and not starting at top). i would get pH closer to 7, if low... re-assess what you feed. calculate ppms, if you can. that will help deduce if there is something going on relative to concentrations or even ratios -- if p/k/ca/mg/fe/zn are too high realtive to each other (whatever their proper ratio is for that specific plant, trial and error), they will compete with each other in ways that will have negative impacts on the plant. could be N.. i see hint of a K-deficiency too. it's a blurry leaf in background, though. pH may very well fix it on its own and oyu don't have to dump anything as a guess into soil for that solution.. then, move on to next option.