There can be a number of causes, if you are a Mac user and you don't use a clean eject app/script some of the hidden Apple cataloguing files can cause this issue.
Songs in the wrong file format such as AAC or OGG can also cause this issue as can some firmware versions.
Start by making sure all the songs on the player are in a supported format, for most versions of this player that means MP3 and/or WMA, however this player has been released under several brand names in several markets and one or two versions don't support WMA.
Also, if you are a Mac user, use a clean eject app such as
FinderCleaner, it's even supplied with an Automator workflow so you can create a menu item and/or folder action for it.