I wouldn't be surprised if this issue is because you're using a version of MB that is at least two years old. There have been lots of changes to MB, including syncing, since then.
If you're using the Store version, I suggest getting rid of it and using the Portable. Install it to a directory like C:\MusicBee. If you're already using the Portable, or the Installer, see my sig to download and apply the latest 3.3 or 3.4 patch.