Add files to library from an external HDD

I connected an HDD from a Windows laptop to my MacBook Pro for the purpose of adding music files to the Itunes Library. I copied the folder on that HDD (let's call the folder MMT) to the location where the files that I rip directly from a CD to the library end up, they're all on my Mac and I can play any of them but I can only get some of the files to show up in the library when I'm in Itunes and viewing songs (or artists or albums). After I copied that folder to my Mac, I was able to open Itunes, then go "Add ToLibrary" from the file menu, and navigate to the folder MMT which resulted in a lot of those hundreds of files showing up in the library when I'm in Itunes (i mean, in the left pane, wether that be songs or artists or albums); However, a lot of the files, which are now on the Mac in the folder MMT, are not showing up in Itunes (I'd have to go to Finder and open the file that way, but Itunes will play them). I think it is the music that Windows created a folder for that is not showing up in Itunes (music that I ripped an entire CD into Media Player, as opposed to random songs). If I would go "Add to Library" from the file menu, then select the entire folder MMT (click "open"), it would copy songs to my Itunes Library (but not all of em), but if I drill down, and click on "open" at a sub folder inside that folder copied from the HDD, to try to copy the music that is not showing in in the library, nothing happens. Any ideas why I can't get some music to show up in the Itunes Library? Some files are wma, some are mp3, that isn't the problem.