Are the roots looking healthy? Do they have any bad smell to them? Shouldn't be an over nute problem, seems like they are not getting enough of something, but it seems like your feeding them enough. Again do you smoke. The reason I am asking about the roots is because of root rott which can block nute uptake as well as pithiam wilt, not sure of spelling. Hope this gives you some more things to alimanate.
Adieu