(I’m beginning to sound like a broken record.)
Ok here goes, coco needs to be watered with a low ppm nutrient solution at least daily. Coco is inert and your plants will suffer quickly if you only give water. It sounds like what is happening is you water with nutes, then the next with plain water, which washed out the nutes. You can water multiple times a day if you like, and if your environment is ideal you’ll get faster growth doing that. But you must feed with every watering. A nute solution of about 500 ppm is a good place to start. Also, don’t let the coco dry out. Then you’ll have the opposite problem, overfeeding due to the buildup of salt in the medium.