Google's John Mueller said on Twitter that Google "never will index all pages." Well, it is Twitter and he only had so many characters to write that.
His response was towards a webmaster who wanted to know a way to figure out which pages on his site Google did not index. So John said, "We never index all pages, so you can'f always fix that. Do you see sites with important ones missing? That'd be a bug."
Meaning, it is perfectly normal for you to have a web site of 1,000 page and Google only index 900 or so of those pages.
Heck, even on this site, which has the best content quality of any site on the internet, Google has only indexed 95% of this site. That is I submitted 20,892 web pages and Google indexed only 19,928 pages.
It would however be nice to get a list of the pages Google has not indexed, like to filter out just those pages.