Cannabis can be kind of finicky about the soil you use, the food you feed it and the pH. I would suggest that you might want to do a bit of reading up on what makes a pot plant happy and productive. You do not want to use any soil that already has nutrients in it or any that has those moisture crystal things. You are also going to need to feed them. Look for nutrients formulated for cannabis--in other words, do not just go out and buy some Miracle Grow or Scott's fertilizer. Get something from Fox Farm, Botanicare, Advanced Nutrients, etc. You do need to pH your nutrient solution to the correct pH. When you say you are using pure water, do you mean tap water (which isn't pure water) or RO water, which will be very close to pure?
It is imperative to keep happy and healthy as they live a set amount of time and don't have time to heal up if they get sick. Everything that causes stress or sickness will directly affect the amount of bud you get from autos.
I'd also recommend kicking the light up to 20 hours a day.