If you notice that some of your pages are not being indexed, do not panic. Google usually needs between 7 - 30days to completely index your pages.
It's been more than 30days
Start your investigation by typing site:yoursite.com into the Google search bar. Give or take is the number of results returned close to the amount of pages on your site?
If there's a huge gap in the numbers then there might be a problem.
Step 1 : Read what Google has to say about the issue from here
Step 2 : Make sure the pages not being indexed have proper meta titles and descriptions.
Step 4 : Once you have verified and resolved all the above, send a reconsideration request to Google. Click here