Pro tip: If you get problem, always check the run of pH. If its too high you got salt build up, if it’s perfect. Your plants are not getting what It wants.
Are your coco coir stable? Some brands of coco coir will give you trouble.
Looks like you got multiple problem, and I got a feeling they are related to what others here have been touching.
Coco is a fantastic medium, why it’s so freaking fantastic is that it’s a hydro grow with “dirt”. So it’s very hard to over water coco. I think your medium have been too dry at some point and since you haven’t flushed the coco to run off at watering you got stress from the dry period and a small build up that’s messing with your run off pH. Giving you a lock out.
Pz 