I am lvl 67 now and have not been able to pick up the ability to make heavy knothide. Anyone know why? I have tried the trainer at Honor Hold, but he has nothing available to me. My leatherworking skill is at 344. I have met several people that have the ability at much lower LW skill.

In Shatt city, in the lower city, is a LW supplies vendor names Cro Threadstrong. He's at the southern end of the lower city near the fruit vendor. It costs I think 5 gold and can be learned at lvl 325 I believe.

You can also buy the recipe from the LW supplies vendor in Exodar, right next to the LW trainer.