That box have good airflow in there? Plants don't do well in stagnant air, they like breezes and ventilation and fresh air. Also how hot is it in that box? Overheating can kill your plant off as well. Maybe a few less light bulbs for a bit till she starts doing better.

Consider buying a larger pot and some quality soil (NOT Miracle Grow!) and transplanting into something a bit larger sized with fresh nutes and things (which will already be in your quality soil so you won't have to add them). I think that would help the plant recover as well.

I won't go into things like ph etc because you say you are quite the newbie but as Rusty said ALL those things are important to plant growing so study up!!

Good luck.