MurrayBill, so if I may suggest before you kill it try something and it involves pH, and yea calmag.
I to grow in coco and I water in a couple of ways.
first I give Ph'ed, nute, water to the sum of 1.25 gallon in veg or 1.5 gallons in flower in a 5 gallon smart pot, waiting till my pots are completely dry for my next watering and next time I water with plan ph'ed water to the tune of around 2 gallons. Now with that said I check my runoff PH (and you should be to at least from time to time). Then I adjust my next watering to straighten out my root Zone pH which is what I would suspect might be a problem.
now you said it has been a problem all along and maybe bad genetics but it never hurts to check your run off of your pH I suspect you might be surprised. In coco add water till it starts to drip and catch the first bit you can and test, this is a small step that could save you a plant. I just hit a problem with a flowering plant at week 5, my runoff ph drop to 4.5 in 1 week and I had to start flushing 6 gallons of 8.5 ph water to hit 6. On my runoff. If you don't flush from time to time you can develop salt buildup which can be a big problem for your plant in your pH
so like I said just something to try.
Hope this helps
Mr.Furley