Your soil has become hydrophobic, It has dry "balls" of soil that the water is just "channeling" past without ever soaking in. You need to add a surfactant to your water and water it in very slowly until the pot is heavy again. Just keep watering a little at a time until it gets wet. I use Yucca in all of my grows. You can use Bonners unscented soap or a commercial non-ionic surfactant. you don't need much but you need to act ASAP because the roots in those balls are dying and the microbes are dying or going dormant. Do not worry about over watering but try to limit the runoff so you don't wash out your nutrients and the micro-biome that is already under pressure.
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Normal watering explained. Never let the soil dry out. Soil and or coco can become hydrophobic if allowed to dry. This means it repels water. This in turn will create dry pockets in the soil and the roots and microbes will die there. If your soil - coco have accidentally dried out use a surfactant to help re-wet it. I like yucca powder. Don't let soil remain soggy by watering too much too often. Root rot, damping off, molds, fungus gnats and other problems start in soggy soil. When you do water
water the entire pot. How to learn when to water starts before you plant the seed. Fill your container with fresh soil/coco and weigh it (heft it) this is the lightest weight and consider it a dry pot. Now slowly water until the soil/coco will no longer absorb the water and run-off begins; weigh the pot (heft it) this is the maximum water, the wettest the pot can get. The difference between wettest and driest is the maximum water weight, for ease of explanation lets just say the water weighs 20 pounds. When the pot loses 10 pounds
(half of the water weight) it is time to water again. Slowly wet
ALL of the soil until run-off begins. There is an art to watering. Make sure there is an air gap under your pot, you do not want runoff to be reabsorbed and if no air then anaerobic microbes can grow. This is a cause of root rot. Coco can be fertigate many times a day if you want to continually present fresh balanced nutrients to the roots. Begin fertigation 2 hours after lights on and end it 2 hours before lights out.
Never use H2o2 unless you are growing sterile. It kills the micro-biome. If you suspect root rot, smell the bottom of the pot, it will be sour. I keep Botinicare HydroGuard in stock at all times, it has saved more than one grow for me. Southern Ag Garden Friendly Fungicide is touted to work also be is many time stronger than HydroGuard. I think it would be easy to overdo it. I don't think this is your problem.
Good Luck!