Mp3/wma Disc 12 Folders Only?

[RNUK]

I've just burned an MP3/WMA disc and put it my car. Works nice (although I'd love to drop the file extension off of the song name on the display) but I only have access to 12 folders out of the 15 I have on disc.

Checked the manual and it says 192 folders max and 255 tracks max. I have a lot less than 255 files and only 15 folders. My first thought it was mixed media disc so perhaps the unit was only reading a dominant format but it is reading both media types. I have a separate CD with some of the missing files on and it shows and plays those fine so it's not a file corruption issue either.

Anyone else come across this? Is there really only a limit of 12 folders?