Backup not working

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Hello: I have been using Norton backup in an external drive successfully for many years. Suddenly I found that after doing a backup and I see the details I see "All files already backed up" but not the filenames list as I used to see. If I use the restore option and I do a search, the file is is not there, although it changes the date and time of the backup set. Please help me solve this problem. Thank you.