first problem is you dont know your ph. second if your feeding if plant food(not designed for cannabis) your going to have problems, possibly root burn which i think your going through. also you dont need to feed them plant food every watering. when you mix nutrients the water becomes very acidic which probably is doing damage to your root system. all of your plants are going to follow the same road as your bad ones. my suggestion is to read and learn how to grow cannabis rather than throwing a seed in some walmart nursery dirt and using nutes designed for the yard.