I have used multiple sitemap.xml for my product pages and my category pages and problem is google only not index my whole site map's link.
/sitemap_main.xml -- URLs submitted:114 // URLs in web index :88
/sitemap_products.xml -- URLs submitted:391 // URLs in web index :34
Why has Google added only some pages in web index?
Simple answer: they don't deserve to be there. They could be thin-content, duplicate content, poor content or any other such reason.
It's Google's prerogative to decide what should, and what shouldn't be included in it's index.
The purpose of an XML sitemap is to allow Google to discover pages that would be suitable for inclusion in it's index, but for one reason or another (back-links, navigation etc), it hasn't found them via discovery/crawl process.
