Hey there! It could be several things, and without much information on your grow space, it's hard to say. What are your high and low temperatures?? You could also try lowering your pH level, 7 is a touch high for soil, you'll want to be below 6.8 at least, with a minimum level of 5.8. Like I mentioned before, be careful not to over water as this will "drown" the plant, suffocating the roots, and therefore stunting growth.