I would definitely suggest spacing your plants further apart, if not, then less plants and stretch and train them utilizing Topping, LST, Monster Cropping, and definitely a trellis net for support and for the biggest yields! The more condensed the plants are, the less light can really penetrate and air circulation is restrained. My theory is if it doesn't make it to the top, it gets chopped! So try to get a very even canopy and spread out thoroughly for the best yields! Watch this guy Hygro Hybrid on youtube, I've learned a lot from his informative videos. If you're looking for big gains with minimal effort, you can definitely get that!
http://www.youtube.com/watch?v=7cxOpGXYEOA&list=PLxqP0ADX5jwFCo7V39mQDNscvd4mGSmo1&index=3