ok, so here's the problem and solution as I spent a while figuring these out. That said, I would never purchase or use the single bevel no mercy again.
Because they are 3 layers on the front third or so, there is the brazing between each layer. On the single bevel, that means that the copper brazing actually ends up as part of the edge when you try and sharpen it. Guess what, brazing will never be sharp. So you have to really take off some metal and get through that brazing and down to the metal. I did this with a wet sharpening wheel, low rpm. It went ok, but takes a while. With a file, you will be working on this for way too long. The lansky course stone might do it, but you will have to really work at it too.
This used to be a bit of a problem with the grizzly heads too, when there was a step in the metal and it caused sharpening issues, they were careless with the brazing spill as well. Now the grizzly heads don't seem to have these problems.
Hope this helps.