WHY do songs randomly disappear? Searching for these is really getting on my last nerve, it happens frequently...

Why do songs disappear? Why can't I find some of them? I know the locations of these files and cannot locate "missing" songs that I purchase via iTunes. I've searched by artist, song title, album, to no avail. How do I recover a file that I cannot find on my drive. Most importantly, when will this glitch be fixed? I'm ready to ditch iTunes because this happens far to often...