I've found that most of my site does not make it to the sitemap.xml and I am still wondering why. I have updated the title, keywords and description fields with custom text. And I see on the sitemap info that most of the pages report a _NO_TITLE_, _NO_DESCRIPTION_ and _NO_KEYWORDS_ red values which I wonder if that has anything to do with them not making into the sitemap.xml
The only page that makes it into sitemap.xml is the one I tested overriding the meta values in.
I am attaching screenshots to make it more clear.