What type of files are they (MP3, FLAC, etc.)?
The 'Album Artist' field isn't universal. In some tagging systems there may be different interpretations for the field being used to store the album artist. In others, there may not be universal agreement on the name of the field to use.
In ID3v2 tagging of MP3 files, the 'TPE2' frame is generally used for album artist, but it was originally intended to be used for 'band/orchestra/accompaniment' according to the specification. If you store 'Album Artist' in an MP3 file using Mp3tag, it may show up in some programs as 'Band'.
In FLAC and Ogg Vorbis, which both use Vorbis Comments for tagging, the most widely accepted field name for storing album artist is 'ALBUMARTIST'. But some programs use 'ALBUM ARTIST' (with a space) instead, and others may use BAND, to mimic the ID3v2 behavior.