There are a number of reasons why Google might not have indexed all your pages. In most cases you just need to give Google a lot more time to fully index your website.
But if you have waited for 3 - 4 weeks and all your pages are not fully indexed here is a list of things you can do;
- Make sure all the pages not yet indexed have descriptive meta titles and meta descriptions
- Make use you are using high quality images and also you have set alt-text for your images
- Make sure the content on the page is more than 150 words and not duplicated. Content is KING with SEO.
- Check the page speed to be sure it is loading fast. If it is not, try compressing images on the page.
- Finally make sure the page is mobile friendly
Once these issues are resolved. Proceed and re-submit your sitemap to Google.
Also check out this guide from Google