Have you tried using a file? And I mean it takes a good one.
My hunting buddy, Sterling taught me to do it. Lay the file almost flat, and stroke from tip to rear. Not a lot of pressure, but you will raise a burr on the back.(eventually). Then just easily stroke the burr off and finish with a diamond stone. We use the folding type for touch ups in the swamp. I have tried stones, and sharpeners and just make a mess, although others have a better touch, and succeed. Griz are not easy to sharpen, but it's our duty to get them sharp, nothing else is acceptable. Stay after them.