Why is a site not indexed?
Let us talk about several kinds of indexing problem and how to identify them. A sad statistic: on average 15–20% of a site's URLs never reach the index. And those are not technical pages. These are URLs of pages that could perfectly well bring traffic and generate leads and sales. To that can be added the problems of partial indexing — where your URL is indexed by Google but part of the page's content is not, for any number of reasons.