Google Indexing Question

I am working on a new news website on WordPress and I realised that Search Console only indexing the Post tags, nothing else. I mean there are categories, posts, author pages etc, but inside the sitemap\_index or sitemp XML in Search Console only the post tags are indexed. I tried to submit the sub-sitemap for posts manually but it came with "could not fetch". Search Console only indexed 1 of the 6 sub-sitemaps. At this point, I have no clue what is wrong, this is the 3rd Wordpress site I am working on and I never faced any issues with indexing. For this reason, the posts which have tags are getting indexed (because of the tags) while the ones with no tags are not getting indexed. This is really painful. Any help will be much appreciated.