Ariendelle is probably on the right track. Here's an experiment to try:
1. Find an album that you know is on disk, but you don't see it in MC.
2. Find, display, or write down the exact path on disk. Like C:\itunes music\Bruce Springsteen\Darkness At The Edge Of Town\<lots of songs here>
3. In MC, go to the left navigation pane, and find Audio > Files.
4. Using the Location Pane, which should be the first one on the left, drill down into your directory structure and try to find the file path from #2 above.
Is it there? If so, click on a song and bring up the Tagging Pane (<right click> Tag). As Ariendelle suggested, look at Artist, Album Artist, and Album Artist (auto). You will probably find something amiss.
If you *don't* see the directory or files when you drill down into the Location pane, then MC hasn't imported it for some reason. I've had numerous corrupted files from itunes, but it wasn't itunes fault. It was just an old music collection that I had on a drive that crashed and I recovered it. A LOT of the files were bad and I didn't know it until I looked (and listened).
I've now re-ripped everything I care about to FLAC and threw away my itunes library completely. The lossy AAC files weren't worth messing with anyway.
Good luck.
Brian.