In addition to the good general settings from @Taomyn , please note that VSS might not snapshot .ost files, and it appears to be the actual Outlook design intent. You can use a search engine to read more on ost vss.
Backup does not contain ost files is a previous forum thread. To be most sure this is it, watch your registry… Though I can’t test here, I wonder if Outlook removes that FilesNotToSnapshot entry when it’s not running?