The most likely problem is that the flash drive is formatted in a way the UConnect system doesn't understand. (Maybe exFAT if it's a brand new drive.) Try reformatting as FAT32. NTFS should work as well. Another possibility is that you didn't rip to a supported file format. Are you sure you ripped to MP3 and not WMA? If so, what bitrate? Variable bitrate (VBR) or constant? You could have issues due to your folder structure as well.
Unfortunately there's no good documentation about what these systems support and there seem to be differences between the 8.4, 8.4NAV, and 12. We're figuring all of this out via trial and error.