I was having this problem, any file I added was not showing up in my library. It was showing up in the Recently Added playlist but was not in my actua library. Here's how I fixed it:
Go to ITunes > Preferenced > Advanced
Notice where your Itunes Media directory is...
quit ITunes, and delete the entire Itunes folder and all the files associted (MAKE SURE YOUR MUSIC FILES ARE COPIED SOMEWHERE ELSE THOUGH)
then restart itunes, dont change any settings, and go to file > add music
This worked for me, after I added my files I tweaked the settings like Don't copy to itunes media, turn off cloud purchases, etc.