You must have at least a Downloader role in a folder in order for it to appear in the list of folders available for syncing. For example, if someone shared a folder with you and you’d like to sync it, you must have at least a Downloader role in that folder for you to see it in your list of available folders. Check with the person who shared it with you and request that your role be changed.
If a file is edited or moved on your desktop computer while the file or the folder it’s contained in is modified, the file might be missing from your list of synced files. Check the local trash folder on your desktop computer for the file.