On Twitter, Google’s John Mueller said that creating additional Sitemap files won’t help if Google does not index specific URLs on your website. If Google indexes only 30 of 100 URLs, it does not help to put the missing 70 URLs in a separate Sitemap file:
That wouldn't change anything. If we're not indexing 80 URLs from a normal 100 URL site, that sounds like you need to work on the site instead of on sitemap submissions. Make it awesome! Make every page important!— John ☆.o(≧▽≦)o.☆ (@JohnMu) 26. Februar 2018
Why Google does not index all of the pages
Google only indexes web pages they find relevant. John Mueller’s says that you have to improve your pages if you want to make sure that Google indexes them.