Plant not responding??!!! HELP

Alrite in my signature is a link to my first girl that I have been working on. Im going to snap a last pic of its 6-8 weeks of vegging. Im really kind of scared of whats going on.

Heres my problem. Over the past few days I havent seen her reach for the light (cfls) like she normally does. I would have to go in there and kind of fan her out with my hands. Now that shes not responding...the lower part of my canopy is starting to fade yellow (smaller leaves). All the bigger/new leaves are fine and green. Is this normal? Should she always be responding to the light?? I mean ive used lst on her again and the stem came back up as if it was responding to it. Weird.

Edit: Ill be switching 12/12 tomorrow.

This baby girl is pretty big btw. I think its an N deficiency. Its been a while since ive been vegging her + lst so maybe its just best to switch up the lights on her. I just need some tips on Nitrogen when I move into flowering since ill be using BMO 1-8-7 flower power..low # in N of course.

Anyone?? Mammath, killa, where r u!!

I checked on it this morning and the yellowing is not getting better. The bottom is starting to brown a bit and dry up really fast. I wonder if I veg'd it to long?? Im sure its not nute burn I feed it 5-2-5 every other watering, but it wasnt quite diluted as I wanted it to be. Either way it doesnt look like a burn. The yellowing started few days ago and is working its way up. Any help would be appreciated!!!!!