After many hours of mowing and honing, the metal at the edge gets worn back to where the metal is thicker. At this point the whetstone will no longer be able to restore the acute angle required for a sharp edge. To resharpen your blade, you will need to hammer the cutting edge, to draw out and thin the metal at the edge, until it is thin enough for the whetstone to be effective again. There are many different styles of peening; not just one. The peening technique that I personally prefer and recommend, is described in my 1SR Peening Manual. It is available as a document below. For a quick sense of what is involved with peening, skip ahead to the videos below.
Special Note for Bush Blades: It is not recommended that beginners peen their bush blades. You risk creating too weak an edge profile for cutting woody stems. Instead, sharpen by first honing them with a fairly coarse artificial whetstone, or a fine mill file or diamond file, followed by a scythe whetstone. Bush blades are also honed at a steeper angle than a grass blade. Hone the bush blade in the field, as you would the grass blade, but at a slightly steeper angle. I like the blue Bregensers for bush blades. When the bush blade needs sharpening, hone it as in this video, but with the Styria artificial stone, followed by a natural stone. If you can imagine a spectrum of edges, with a razorblade at one end, and a hatchet at the other; the grass blade would be closer to the razor, and the bush blade would be closer to the hatchet. So the edges are treated and shaped accordingly.