Hello,
Usually I do backend but I had a chance to work on a new site. I faced several issues with indexing it by Google.
I started with http and then I switched to https. And the sitemap was changed that "dangled" some pages in statistics by Google (these pages were in stats but absent in the new sitemap). So, maybe my sitemap should contain https pages only? I'm still not sure.
And my sitemap doesn't contain pages like www.*, but Google still tries to index them. I added the www record to my DNS, but it seems this didn't help. Then I added these page to my sitemap, and I'm still waiting for the changes.
Some pages had wrong canonical links from the pov of Google - they had clones and Google asserts that these clones are canonical. But those pages don't have ssl certificates.
All in all, I faced a lot of points with the indexing process by Google.
