Plex essentially has two ways to identify your albums:
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By folder/filename structure - for this to work, the folder/filename structure must conform to Plex’s guidelines.
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Embedded metadata - for this to work, your embedded tags must be accurate. This How-To article can help. The symptoms you describe are commonly caused by incorrect/inconsistent tags, especially in the album title or album artist fields, so pay close attention to the How-To with regard to these tags.
Once Plex identifies the album, it will attempt to match it to the correct entry in the MusicBrainz database, which will allow retrieval of artist biographies, album reviews, concert info, lyrics, etc.
One thing you should be aware of: Plex has not implemented support for the Composer tag, as well as a few other tags which are essential for properly organizing classical music. My personal workaround for this problem is to keep my classical music in a separate library from everything else, and to modify the embedded tags improperly, to account for Plex’s deficiency (using the Composer as Album Artist, for example). This also means my classical music is in a separate folder on the hard drive.
Until Plex resolves this deficiency, having classical and other music in the same library is not practical, IMO. You may want to add your votes to one or both of these feature suggestions:
By Composer
Tag support for ROBUST music library organization
