First of all, please make sure there are no filters that might be incorrectly configured in Backup Properties->Filters/Customize. To verify that, disable the filters you made and calculate the statistics for the next backup.
Second, please uncheck the Smart file scanning mode option from Backup Properties->Advanced page. Again, you can calculate the statistics and see if the files in the sources are found to be backed up.
Third, if the source files are located on a shared network drive and the backup is scheduled, please make sure that enough permissions are granted to access the shared files even if no user is logged on.