BackUpwordpress does not add the files to the backup

I have noticed that the plugin seems to work but in the backupfile to loads down per Email is only the backup of the database, not of the files. Then I noticed that the files are obviously backed up, too. But not included in the backup. This is serious error and should be changed as soon as possible, otherwise the Plugin is USELESS.
This happens at 4 different sites, therefore not specific to one site.