This may or may not have anything to do with it, especially since you're only a couple weeks into growth, but if you plan to continue using distilled water or water treated with reverse osmosis, you need to supplement for the calcium, magnesium, and iron that the plants won't be getting which is normally found in tap water. Look either in hydro stores or on the internet for a product called Cal-Mag Plus, it'll take care of all those needs for you.