xasharma you should always summit the /sitemap.xml URL to search engines. The individual child sitemap files are not guaranteed to work using direct access.
If you share the fully qualified URL with your domain name we could take a look at the actual file to see if it's broken or incomplete.
Again, please share the version of FoF Sitemap you are using. There have been some major changes in the last versions (the ones that require PHP 8+). If you aren't already on that latest version, try updating.
