Nobody can tell you anything without proper system info or other technical details like what files you are trying to sync from what program.
File Sync Links that may help... all the links I have, since I don't know the cause of your specific problem
-Size Limits https://forums.adobe.com/thread/1488242
-sync and email link http://forums.adobe.com/thread/1427516?tstart=0
-Phantom folder problem https://forums.adobe.com/thread/1490445
-an overview of assets https://assets.adobe.com/files