The cause has been discovered, Whiteflies, turns out they suck the life out of the leaves turning them yellow and then they crap something known as Honeydew on the leaves which is a clear sticky water soluble goo that blocks the leaf from absorbing CO2, shutting down photosynthesis and stunting growth.
The solution is to spray the plant and the bottom of the leaves with clear water and then spray them with a watered down solution of dish soap, it doesn't kill the Whiteflies but they are repelled by it and move on,
One thing to note is the Whiteflies are resistant to most pesticides which I don't like using to begin with.
With the insects gone the plants are making a nice recovery.