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Please tell me why google is not indexing all sitemap pages? i have submitted 23,620 pages but they have indexed only 618 pages!!!!

Because the rest of your content is worthless or duplicate.

Google usually take time to index a large site as they have a "crawl budget".
But I agree with Kristy above. You probably need to assess your site for duplicate content issues and put in a whole bunch of rel canonical meta tags everywhere.

Maybe your content quality is poor and duplicate.. Google tends time to choose and pick what to index if they found a duplicate in your site they will choose the original unique one first to index and maybe index yours later on in certain ways. Depending how you cope for it.

The best thing to do is test your site.
Speed
Content
On page seo
Off page seo
This are basically what Google looks for that is to have a high quality of the list above.

It usually takes time, wait for sometime and try using good indexing service.