There could be several reasons why Google doesn’t show all the pages from your website in search results but the main culprits are likely:

Spider Blocking
If GoogleBot (Google’s crawler or ‘spider’ software) can’t discover these pages, then they won’t be in search results. Certainly less common these days, ‘spider blocking’ occurs when the links to the hidden pages are not able to be followed by GoogleBot – ie the door is shut and GoogelBot cant get through
Typically causes are:

Resolution:
Make it easy for search engines to find the pages you want them to:

  • Create and register an XML sitemap with Google
  • Add a robots.txt that points to the sitemap