I just sharpened a couple of Shun's for a chef. I don't know the history of the knives but they were pretty worn. On both of the knives, after more than fifty strokes to the front third of the blade, I barely raised a burr. While the rear two thirds raised a burr after about twenty. I know the heat treatment can get ruined, but for just a portion of the blade? What else could prevent that portion of the blade from taking an edge?