I might guess a couple of things but this are simply guesses. For one I thought I saw a few pages back that your ppm were 1800. That is a little high and might be blocking out some nutrients or causing stress. Others can probably chime in that run there's higher, but mine is usually around 1000-1100 with an EC at about 1.5 or so. Depending on the scale you are using it sounds like you are around 2.5 to 3 EC which is pretty high, I always thought you shouldn't be much higher that 2 EC. When I ran mine higher I noticed most In House strains that I was running was not as happy as others. ppms are tough though cause it's not so much what the number is as what it is in the water that is making the number so high, some things won't stress a plant as much as others. Are you using a RO filter for your water?
Also there is the law of minimum which could be a problem for it.
"Law of minimum:
According to Carl Sprengel who developed this principle, the laws of minimum merely state that plant growth is not determined by total resources available, but by the scarcest resource. This means that while your plant may have abundant sources of Nitrogen, Phosphorus, or Potassium, your plants may lack micro nutrients such as Boron, Molybedenum, Calcium, Magnesium, and others thus inhibiting and stunting growth limiting your plants full potential."
While you may be giving it enough of most of the things it needs, you may be missing some trace minerals that the plant requires to process the N,P,K and Cal Mag. I'm not that familiar with Boatanicare stuff now but I think that their Liquid Karma might help you some if this is a problem. If I remember correctly it contains seaweed which should have some of the stuff you might be missing.
Hope you figure it out though.