That means you have multiple songs with the same title and files and MobileSheets can't distinguish between them to figure out how to match up the songs. I know you are saying there is only one song, but that can't be the case. There are most certainly two songs in your library with that same name. Please go to the Songs tab, scroll down and see if you can locate both copies. If you can't, then I may need to work with you to look in the database file itself, because MobileSheets is seeing multiple copies of that song. It can also occur if your two devices each have a different copy of the song and you've assigned different song IDs to them. MobileSheets will match on song ID first, so if you've been assigning song IDs without realizing that, that can cause major issues with the synchronization feature.
Mike
Mike