if its only 1 of the 5 that is affected and they are all in the same media and recieving the same feeds i wouldnt sweat it too much. It may just be a dodgy one. I think they are all looking great just do what your doing.
On the flip side with it being the oldest it is likely to have seen the most nutes and you media may be seeing some PH issues. Give it a good flush followed by a light feed.