Sansa 240 does not show all songs when connected to pc.

I have a Sansa 240 which is now approx. 95% full of songs. When I look at the songs I've added on the mp3 player itself, there are many more songs on the player than are listed on my computer when I synchronise the player to view the songs. It appears the player is not showing all the songs when it's connected to my computer. How do I resolve this so I can delete some of the songs off my mp3 player?