I learned from growing a rose garden some years ago is that you just have to let some plants go (die). You do the best you can for them, and it doesn't always work. It's okay to yank that sickly rose plant out, and start over with a new cultivar.
Similarly, if your other cannabis plants in the same nutes do fine, this is just a weak plant. They're not clones are they? Regarding your photo, the plant looks yellowish to me (nitrogen, sulfur, iron possibilities), and the spots remind me of calcium deficiency.