Check in you previous library file folder on your computer to see if that has a more recent version of the iTunes library that you want. Also you can import the items that are missing by dragging them on top of iTunes to import them in.
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Thanks for the information Norm. Since the new music I added (since May) was in my iTunes folder, I just manually went through the folder and added the new files by dragging them into iTunes. I added about 400 songs.
Also, the newest library file in my "Previous iTunes Libraries" folder was from May. Does iTunes automatically create these files, or is there a way to do it manually? It would be great if these files were created weekly, especially if the user (like myself) is always adding files.
Thanks for your help.