I have this issue (among many) with iTunes, most recently today.
Here's one thing I discovered that helped me fix this issue with certain files:
iTunes apparently periodically decides to reset itself to the default "where to save music to" folder for importing music (from digital files or CDs). This probably happens when it updates or something.
In my case, I generally use a different folder than the default location, and it's stored on an external drive.
If iTunes has decided to reset the import location for music to the default C: location without telling you it's done so, you won't notice anything different initially. But if you had at some point in the past specified a different location for saving music to, what ends up happening is your music becomes fragmented across the two different save locations. In my case, I had a folder for the same artist in both the folder my music is normally stored in (on the external drive) and the default folder that iTunes inexplicably reset itself to (on the C: drive). This seemed to cause iTunes confusion, even though the tags were exactly the same in iTunes itself.
I'm probably not explaining this well, but the moral of the story: file location (of the .mp3s) can contribute to this issue. Check to make sure all your music is saved/imported/stored and saving/importing/storing to the same folder, and that your library isn't fragmented across multiple locations outside of iTunes. Then re-import anything you had to move (and re-tag if necessary). Once I got everything by the artist in question back into one place on the hard drive, iTunes was able to group everything properly again.
There are other things that can play into this I'm sure -- what settings you use for music management, how you've set up iTunes to handle syncing, etc.-- but actual file locations are just another thing to check on that could be contributing.