Song List Difference

In Windows Media Player, my song list shows 1827 items, but my Sansa MP3 player only show 1527.

I keep the same number of songs on my computer and MP3 player so why the difference? 

Did the Sansa stop recognizing when I kept adding more songs? I had enough memory to keep adding them.

The missing 300 files might have ID3 tags in a format the player cannot read. Or possibly those 300 are in an audible format the player does not recognize.