This is really annoying; it worked for all the other songs on the album, but one song will stay at the same volume no matter what the gain I set it. It also shows the song as being at a very low volume, although it is not the case. Does anybody have a fix for this??? Thanks!

The file is probably corrupted. Got footbar2000? In this case you could try the option "rebuild mp3 stream" in the context menu under "utils" when right clicking on the file. Otherwise re-encoding the file should work but will reduce sound quality. In case you choose to re-encode the file, I would choose a high bitrate so not to loose to much quality.