Dropbox sometimes needs to be told what type of file you're downloading, especially music or video files.
For example, here's a problem I've noticed: If you use a file manager to create a subdirectory in your Dropbox directory and manually add an *.mp3 file into it, again using a file manager, it may not show up in Dropbox and the file might be converted into a different format. For example, I've done this and the *.mp3 file was converted into a Google Reader file format. Could not be seen and when tracked down manually, could not be played, even after changing the file extension.
Here's the solution: Go into the Dropbox application, Menu, choose "+ New" to create a new Dropbox directory. Then, open the Menu again and choose Upload. Choose the file type, eg, Audio, then choose the program used to play the file, eg, "Select music track Music. Then check off those files that you want to upload. Now everything will work properly and will be available immediately.
This may not be exactly what you're experiencing, but hopefully it will help some people who have "Dropbox Questions on a Thunderbolt."