Coco can be treated like a soil where you can throw some amendments in and give it dryback waterings, and coco will behave better than peat with aggressive dryback cycles if you do it right. Coco can also be treated like hydro with no amendments and being fertigated daily / multiple times daily with liquid nutrients or dissolved nutrients in your water. Sounds to me like you're applying a soil style. During the drybacks even if it's been washed, coco can start absorbing and changing the nutrients. It takes your calcium, magnesium, iron, manganese and ammonium and locks it inside and dumps potassium, sodium and hydrogen ions in their place. You can counter it by upping your cal and mag feed a little and backing off your K value and try not to let the weight drop below 80% of "just watered." Alternatively, if you can use liquid nutrients, water multiple times daily with a light feed. If necessary to ensure good oxygen, throw some perlite in there. Coco is one of the best mediums for growing weed and while it can be prone to behavior problems, they're manageable once you get a general gist of what's going on.