Did the other tracks you imported upload to Dropbox? And do you have a MIXO folder in your Dropbox?

Could you please try the following:

- Go to Settings > Debug > Delete Upload Queue, then go back to all tracks, select the tracks you want to upload, right-click and select Upload Track(s)

Let me know about the above and if the fix had any effect for you, if not I'll try and help further.

Thanks

Jon