Did you rinse and dilute the old coco?
drying actually increases concentrations.. washing won't help if the CEC have gone the opposite direction and have too much Ca/Mg bonded. You don't need to buffer when re-using.. just wash it well and go. if anything is separate, likea layer of hydroton, may want to sanitize that.
anyway, my 2 cents is it is an overabundance of one nute or another -- imbalance. Also ensure pH is 6-6.5, or whatever your ferts instruct.