What is happening to them please?

Nutrients are complicated and anyone can easily tell you to do the wrong thing while missing just one piece of information or simply overlooking something else etc etc.. So, always consider your recent behaviour and intuition. Verify stuff that i or anyone else suggests before doing it. Numerous factors are important, but if you pay attention to these three things you can avoid most problems: Concentration, pH and ratios of primary/secondary/micronutrients whatever... For example, when you know the general PPMs of what you feed, you can easily rule out causes with similar symptoms. Avoids making a problem worse by exacerbating the lockout of other important nutrients. Get a Symptom leaf chart. Google or one is linked in my main profile to a picture hosted on growdiaries.com/ -- no external site. I also have a nutrient spreadsheet there, if you want one. No guarantees it works for liquid nutrients, but i think i have specific gravity worked out correctly (this is a google drive link). A professional AG brand would likely have this info readily available - at least as instructed on label for verification of the spreadsheet output, or find a nutrient ppm calculator on the web somewhere. Watch the youtube vids too. So, based on what i read and see in your diary, Could by Mg deficiency. Could be lockout of Mg. I'd lean toward lockout. you'll want to inspect the labels of the products you use. figure out ppm of the Mg you provide. If it is <80ppm, add some magnesium (epsom salt is an option if sulfur is not already too high) Your leaves are lush. Could be the photo, but certainly don't look like they lack fertilization. Just a balance issue, because it is highly unlikely they left Mg out. Telling someone what to do with a nute problem is like playing with fire. So, do what you are comfortable with while armed with much more information than what is given, and take a conservative action before anything that could cause collateral damage.