Yellowing of the leaves could be many things. Could we get a little more info, like what kind of lights, nutes, etc, etc, your using.
I will say just from the picture, looking at the plant in the background, it seems to be a little more light green than it should. Your lights may not be strong enough, or they may be too high above. But that may have nothing to do with your original problem, that was just my first impression from the photo.