In the Miramar theme, the interstitial is the block of 3 posts that breaks up the content of the Home Page feed. If you cannot see it, there are a few ways to troubleshoot:
Did you supply content for this section in the Customizer?
The section requires that you provide imagery and links for the three boxes; it will not auto-feed in any content without you providing these pieces. You can upload content for the three boxes by going to Appearance>Customize and then selecting the Interstitial tab.
Did you set the interstitial to appear after the total number of posts on the page?
Our themes give you the ability to dictate where you want the interstitial to appear. However, the number of posts it’s set to appear after is impacted by how many total posts you want to appear per page. If the former is bigger than the latter, the section will not appear.
For example, let’s say you have set up your theme to display 10 posts per page, and the interstitial to appear after the 11th post. Because there is no 11th post, the section will not appear.
You can change where the interstitial appears by going to Appearance > Customize > Interstitials and adjusting which post the interstitial should appear after. This setting is here, and is called Display interstitial after post number: