Hello Raviraj,

the problem could be related to scalability. If you have added data sources including too many (thousands) of links in the sitemap the generation could take a lot of time and not complete.
For the HTML sitemap normally you should include max 5000-10000 links otherwise the browser hangs to render the sitemap tree to the end user.

For the XML sitemap you can use the precaching feature if for example the sitemap includes 100.000 links.

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Best regards,

John