If you appear to be missing files on your online account, you may need to check that the files you wish to backup have been selected manually. To do this, simply click Backup Settings on your application. Once clicked, select the Tab called Backup Selection. Then select "Manual Selection" - This will pop a window, where you are able to see what files have already been selected as well as selecting new files to backup.
Once complete, simply click OK, followed by apply.
Then Simply go back to the App and select - Backup Now, this should now backup the files that have just been selected in addition to ones that were already selected.