Not just you David, Black Walnut can be a real pain at times, as I said in another post, some trees in a species have just different wood, where some work easy others do not, and Walnut is one of those that does this often enough, it is after all not a hard dense wood and that is one of the reasons it act sometimes more like poplar or willow, adding a grain stiffener like wax or oil or even water will help get a better result with turning it.