133 songs listed when plugged into the computer; only 90 available to play - where did my songs go?

SanDisk MP3 players Sansa® Clip | Sansa® Clip+
2

A couple of possibilities:

  1. The ‘missing’ songs are in an un-supported format (like Apple’s proprietary.aac or .m4p).
  2. The ‘missing’ songs have incomplete, wrong format or missing ID3 tags.