Like others have said, check your runoff ph and ppms-it's not super reliable but it will tell you if something is way off. To "flush" in coco means simply to pour more of your ideal nutrient solution through the media, not to flush with plain water. Flush it with the PH and PPMs you want for your plants. Coco needs to be fed regularly to keep salts from building up and to prevent PH swings. You might want to consider feeding multiple times per day with a lower EC like 1.0, with plenty of runoff, and I bet your issues improve.