Alex has videos on prepping, modifying and using nagura. Round off all the corners and edges, I round at least one corner to about the radius of a dime or more and smooth the surface. Cut grooves with a hack saw if you have sticking issues while using a flat side. I don’t cut grooves and just use rounded edges so you are using a smaller surface area and use a rocking motion so you are not creating a flat on the nagura.
Try all 8 edges before giving up on the Tomo. I would also switch to a diamond plate if you are still having issues.
When trouble shooting a problem, begin by eliminating the most likely suspects.