When processing a photograph and then convert to DNG format I try to use the tool to compare before vs after in develop mode but nothing happens, the picture above is the same as the next, and just happens to DNG files
DNG is just another 'raw' file format. In lightroom it will look exactly the same as any NEF or CR2 file that you 'convert'. The Develop settings applied to the original will be the same applied to the DNG. Nothing changes except the filename suffix.
You can choose any step in Development history to be the "Before" image appearance, by a right click on a step in the History panel