Brother what kind of light are you using ? That 6 inch height may be killing her and not much in the time line of information in this journal ..Yes using old soil can leave it lacking a diverse amount of nutrients or a build up of one or another ..both can cause issues and of course the over watering let her dry out a bit don't be afraid it's a weed that can recover very fast and you'll be fine .. maby tone down the amount of nutes your using cut back a bit as well ...cheers