:smoking: Ibz-- sorry for the flamer mate, I didn't get here in time, but I see others did!
...so, coco, a rather specialized medium, not an easy learning curve to grow in, but once dialed, it's excellent... as you've read, your pH is likely the culprit here,.. coco should be in the upper 5's; in a way, it's much like hydro, for pH and nutrient management... Also, coco has peculiar cation exchange capacity characteristics, with Ca in particular, and Mg and K to a lesser extent,.. have you done your homework on coco? Biggest, most common mistake I see, is peep's treating it like soil, or even something like Promix-- soilless,... (that's peat based),... this means unless you have dedicated coco nutes, you'll need to add extra ca-mg every time you water... it's fairly complex and lengthy explanation behind this, so research if you haven't.. in a nutshell, coco is a Ca hog, and will bond it to it's particles and hoard it until sated, keeping it from the plant until so,... maintaining the ion balance in there for proper nutrient availability is the key,... alas, I can only offer you what I've read and learned, since I've not grown in coco yet, save one mini-pot! but Duggy, Arty and med here have set you right with the steps to take for correction,... I also recommend a root tonic, like Superthrive, Jump Start, that sort of root stimulator and shock treatment,...