This is the id3 name of the field that is shown in MP3tag (normally) as ALBUM.
See the help on field mappings:
Then I would run a consistency check over all the files (no, it does not matter that they play allright), see this thread for some useful programs:
I recommend MP3val and MP3diags.
Once you get no other errors in MP3diags than complaints about low quality audio, padding and missing replaygain data, try the files again.
If you do not see something but it should be there, check also the settings in MP3tag at File>Options>Tags>Mpeg which tag versions you read, write and delete.
A good idea could be to get rid of APE tags.