This particular song that does not play on iPod could be doing this because it is not in a format adaptable for the iPod. To sort this, highlight the song and right click it to choose the "Convert selections to AAC" option. By doing this you will have converted it to an iPod format therefore play.
if it is greyed out you need to plug in your ipod and go to music and redownload it. if you have selected only certain songs for your ipod you just need to check it then unchecked it and then hit the button in the lower right corner. (I can't remember what that button says)