We get so caught up in GMB, Citations, Content etc... that we can forget setting up the basics that help Google Crawl and Index. Take some time to check old sites. Your bound to find a few things you can improve on.

The Definitive List of Reasons Why Google Isnít Indexing Your Site

This is the most essential SEO feature of all time. Google must index your site in order for your site get any organic traffic from Google. If your site isnít indexed, you are lost. No one will find your content organically, because itís not part of Googleís search index.

The first step to fixing an indexing issue is diagnosing the indexing issue. This list will help you do just that.
Iíve roughly organized this list from most common to least common. You can work through the list from top to bottom, and youíll find your cause and cure.