Why can't I locate a file on my Creative Zen?

When I transfer a particular mp3 file to my Creative Zen I am unable to find it on the player. It is viewable from Windows Explorer as being in the Music folder and the change in memory capacity seems to indicate it is there, but it is nowhere to be found in the player menu. I have even tried renaming the file to see if there was something about the name that it didn’t like but that didn’t help either. I have recently reformatted the player and installed the latest firmware. Any suggestions will be gratefully received.