Well, in my experience you just get some plants that don't want to cooperate for some reason. This young lady looks like she is not enjoying your 600 watts of light due to the fact that she is not willing to become a voracious consumer of nutrients.

You may be on your own here... I know its not usually a good idea, but you might want to think about cutting back on the light a bit. Sounds to me she just isn't hungry enough and the leaf browning might be due to a bit of burning. Not sure though man, you are getting a better look at her. Going 12/12 and backing that bulb to about 3-4 feet might not hurt. Thats my best guess, sometimes "weeds" need to work for their dinner to become a more aggressive growing plant (ie. letting roots go unwatered for 2 days etc.)

Keep me updated