I presume these files are within the iTunes library. Open iTunes & check those that are not showing up in the LW library. Are they mp3's or another type. There's different types of mp3 & there's also mp4 which usually uses m4a or AAC extensions. (By different types of mp3 I mean there's wma, rm (realplayer files), or ogg. That version of LW might have difficulty recognising some of those newer formats, particularly the windows ones.)

Those that show up as Quicktime files, open them up with QuickTime & hold down both apple & i keys. What info does it tell you? This 'might' give you a clue. Also just selecting them on the desktop & using apple & i keys might also give you some clues.

There are utiltities that can tell you all the details about the files but they're usually shareware.

If you encode your own files do you use iTUnes?