Why is this happening?

Kush420 is right. Even using well water can potentially cause problems. Salts build up & cause "lockout" & every plant even if same strain from same breeder can grow very differently, that will show u the quality of that breeder too

Hello! also I have had the same problem in the past .... I wanted to tell you that sometimes the excess of some elements can block the absorption of others even if it does not overfertilize especially it could show you deficiencies or excesses similar to other deficiencies that they can lead you astray in the analysis of your problem such as an excess of phosphorus could block the assimilation of another nutrient and the opposite .... each plant reacts differently to problems ... so we are always careful to use fertilizers because sometimes it's better less than too much! You could do a test by slightly lowering the pH or lowering the fertilizers for a few days .... good cultivation!

Hey @ BigDaddyK! I think it could be a calcium problem. If your waters initial EC (before applying the fertilizers) is above 400EC you should consider using osmosis water or mix with mineral water for lowering your initial EC and then increasing it by adding CalMag. Good look!

I mean from those leaves its a lack of phosphorus. The die off in the middle and curl over. I get this if the nutes or uptake is imbalanced, or if a pot is too cold usually.(is one closer to the intake vent?) Is that one feeding on the same water as all the others? I mean its either eating way more and getting deficient, or way less and getting lock-out/overfeeding, if it is the same. Flush it and feed 1/4 strength back in and see where its at in 3 days, looking better go back to normal feeds. looks worse maybe you actually need to feed more.

@BigDaddyK Hi mate I dunno but I did have a plant awhile back that had a major calcium deficiency that looked a bit like that I see your already using calmag tho Strange one