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
all4me Reviewed by all4me on . WTF!!! Hi every one, Dont know whats goin on this time round and its doin my nut in! So here goes........ plants are 3/4 weeks old from clone.Pre ph`d rockwool cubes 600wtt HPS-18/6. NFT,and they`ve been in the system 11 or 12 days. The pump is just a light trickle running only when lights on. Temps are 81 lights on,not too sure lights off temp,but its in family home.... 125mm intake and exhaust rvk`s. On constant.Humidity 40% On the second day of them being in the system I changed the res and Rating: 5