If you have issues with Google indexing specific URLs on your site, Google says putting those URLs into new XML Sitemap files won't help them get indexed. So for example, you have a Sitemap file with a 100 URLs but only 20 of those URLs are being indexed by Google, taking the other 80 URLs and adding them to an additional Sitemap file won't get Google to index them.
John Mueller of Google said so on Twitter. He said it "wouldn't change anything" to take this approach. Instead he said, you should work on making those pages of content awesome and then that would be a reason why Google would index them.
Here are the tweets:
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 â.o(â§â-½â¦)o.â (@JohnMu) February 26, 2018
Have you tried this approach?
Forum discussion at Twitter.