Hello Peter,
So, by default the Sitemap plugin have 3 different ways for rebuild XML files in the sitemap:
- you update manually via Admin panel > Common > Refresh > Update sitemap.xml file = Rebuild. It will always remove exist sitemap files and build a new.
- via cron jobs. If your crontab configured properly that cron file will check XML file: it will create it if files missing; it will updated if last date of modifying files more then 24hours
- via frontend: if files is missing and get a request your-domain.com/sitemap.xml the plugin will create it. It wouldn't update files if they are exist already.
If your cron doesn't update old files, that maybe it have a errors.
You can check it manually:
- rename file /cron/.htaccess to backup.htaccess for example
- run cron script via browser your-domain.com/cron/index.php
If it return empty content that cron works properly and doesn't have errors.