Why won't Media Monkey add one particular folder of mp3's?

I’m using the latest and greatest version on Media Monkey (free version) and it won’t find the mp3’s in one particular folder in my music tree. It can see all the other files in the tree and the folder shows up when I click

Add/Rescan files to the library…

1. I have full control over the folder and all the files in the folder.
2. The files play in Windows Media Player.
3. The files play in Media Monkey if I right click and play from the context menu.
4. All the tracks are at least 2 minutes long and over 5MB long and Media Monkey is set to ignore files shorter than 20KB and include all files regardless of length.

There was an issue in that the that the genre of the tracks was set to “Classical” and the option that allows you to browse the classical music independently of the other music isn’t enabled in the free version. It’s a Gold version option only.

I hadn’t spotted that my other classical music was also missing from the library (I have rather a large library). Once I retagged the music with a different tag and tried to add the files again it reported that it added the tracks, but they still didn’t show up in the library.