Sometimes plex sees two discs, sometimes it doesn't

General Discussions
1

I’m having an issue where sometimes Plex will read two discs in a set and keep them together, under one heading, with a disc one/disc two ect tag. Other times it will treat the two discs as separate albums. How do I get plex to be consistent in this? The collections are tagged in the same way (using mp3tag) so I know it isn’t because the tags are different.

2

I am going to do (again) a bit of copy/paste of posts I did in the Plex Pass only forums:

Check your embedded metadata.
The AlbumTitle tag must be exactly the same for all files on both discs, which means it must not contain a disc number.
There is a special meta tag reserved for the disc number.
Also, the AlbumArtist tag must contain the same for all titles on all discs if it shall be recognised as one album.

Plex is even able to recognise a multi-disc release even if the files don’t have embedded meta tags.
But even if your files are indeed tagged, I recommend you to follow this convention for subfolders and file names:

Music /
   Artist /
      Albumartist - Albumtitle /
         Disc 1 /
            1. trackartist - tracktitle.mp3
            2. trackartist - tracktitle.mp3
            ...
         Disc 2 /
            1. trackartist - tracktitle.mp3
            2. trackartist - tracktitle.mp3
            ...

do note the distinction between Albumartist and Trackartist.
There are separate metatags for these. You need to fill both.

This then provides a means to deal with sampler albums:
The typical sampler simply uses as Albumartist Various Artists
And that is what you should use too, since Plex treats ‘Various Artists’ a little different. (the standard album sorting mechanism is by release year, descending whereas for Various Artists it is alphabetically, ascending)

If you are on Windows, I recommend you to use mp3tag. It can read and write metatags for all the common music fileformats + mp4/m4v
It also supports deriving metatags from folder/filenames and vice versa, which is excellent for bringing your collection into shape.

Also don’t forget that you need to ‘dance’ if you edited the meta tags or renamed the files after Plex has mistakenly recognised an album.
The Plex Dance (for music, step 4 can be omitted)

3

Thank you for the very informative answer!! I’ll check this as soon as I get home!