First I'd need to know how it's being grown, nutes used, and ambient conditions. But first I always check for bugs, which I don't think is your problem. Then I move on to other areas, but without having any knowledge of your conditions, I have no clue... Does look a little dark on the green, could be the start of a (P) def. Are your stems purple? Could also be the start of a PH issue.
That's about all I got for now...
GS