I had this issue with my Durban Poison... I had tried cal/mag, flushing, minimum nutes for a month and everything else! In the end all it took was half of a Zinc supplement for humans, Zinc Chelate! You won't recover from the damage but the claw will no longer show in new growth and it will have more supple leaves instead of the leathery and brittle feel I bet you have as well. Good luck, I hate the hard to diagnose problems too...