Overwatering. Let the soil dry out, then transplant to a bigger pot but you'll need some potting soil and mix in a cup or two of powder fert like foxfarm's and you can probably stop using the Grow Big. Use of liquid ferts is fine if you know how much to give, how to control it, but since it's a liquid I'm guessing it might be more for hydro. The powder ferts work great in soil. There's no guarantee that your nute schedule is correct to use because it's not easy to meter soil fert strength. But if you put time release ferts in soil you shouldn't have to worry about ferts anymore.