iTunes will not add some mp3 files
You can try to fix your mp3 file, there are some great software for that.


  MP3val is a small, high-speed, free software tool for checking MPEG
  audio files' integrity. It can be useful for finding corrupted files
  (e.g. incompletely downloaded, truncated, containing garbage). MP3val
  is also able to fix most of the problems. Being a multiplatform
  application, MP3val can be runned both under Windows and under Linux
  (or BSD).


Src : http://mp3val.sourceforge.net

I don't really know if it's because your mp3 files are broken, but it's possible. Perhaps QuickTime and VLC are more permissive than iTunes...