I have one mindmap that doesn't appear to sync/backup to Dropbox. The "1" unsynced indicator stays after selecting (and getting visual confirmation). Wait for 5-10 minutes. Still there. No idea which miinmap doc is not syncing because it is not indicated.
Could you try to move the map to your local storage by going to the file list, selecting 'Edit' from the top right corner and then tap on the file. Then choose 'Move File' and select the 'Local Files' > 'Files'?
If that works, try to move the map back again to Dropbox using the same method.