itunes not saving changes.

This topic has been discussed before and someone responded to the original poster as though the issue was resolved with an OS upgrade. However I am having this problem even with the current OS. When I add songs to itunes, even though the songs are on my hard drive, sometimes they import, sometimes not. I wait for awhile after either dragging and dropping or copying and pasting them. The files are mp3s, but sometime itunes just isn't saving them when I quit itunes.