Flat grind knife always has a secondary bevel, look closely and see if you can see it. If you use stones, diamond or whatever rest the flat on the stone then raise the back edge a little way up maybe and extra 10 degrees, hold that angle as you sharpen, then do the other side.
If you want an easy way or if you don't already have stones a spiderco sharpmaker or similar is ideal fro sharpening knives with secondary bevels.