If he means MP3Tag, its probably one of the best and most reliable pieces of software that I regularly use - I liked it so much that I donated.
Got rid of all my album art issues using MP3Tag by embedding art in the mp3 files.
As for your problem, I have found that slight differences in spelling of albums, different album artists (Should all be various artists) or any other accidental differences such as genre etc will cause the problem.
Have a look at the tracks in WMP advanced tag editor, check the different tabs to make sure that all the generic info is identical for all tracks - WMP can be a bitch
