Backup file sets not complete

I just started backing up 5 websites with UpdraftPlus over the weekend and today downloaded all my backups to see the contents and check things over. They are configured identically to backup the files and database daily to Google drive. In looking over the backups I see that in some cases not everything is backup up. A complete backup set (as I understand it) should contain 5 zip files for – db, others, plugins, themes and uploads.

One of my websites yesterday had all 5 zip files but today had only 3 – plugins, uploads and themes. Another couple of sites didn’t contain all 5 zip files too.

I’m confused – shouldn’t each day with a full backup have 5 zip files associated with that backup? If not, why and how do I resolve this?

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