Sitemaps help the search engines find your urls. Valuable content convinces the search engines to index those discovered urls.
If you do not have significant unique content on each page the search engines will think those pages are just duplicates and not index them. To increase the amount of pages that are indexed work on your internal content and external websites linking to you.
It is rare that G will index 100% of what is in your sitemap. We have improved our indexing to around 75% by focusing on internal linking. Essentially we are trying to get some of our deeper content more exposure within our site. Seems to be helping.