Hi,
It may be that Google needs more time. Though, it could also be Google needs to recrawl it. Can you confirm if you use Inspect any URL that the issue is resolved? This will force Google to update the index. This guide explains more:?https://kb.yoast.com/kb/fetch-as-googlebot/.
If Google still fails to show it, unfortunately, it’s entirely up to Google to decide what to show in the search results. Apart from the schema markup for the primary image, there are other factors (i.e. authority, context, etc.) that Google may take into account to decide whether or not to display an image next to your pages.
As our plugin is generating the schema markup as expected, we recommend contacting Google for further assistance by making a post on their forum here https://productforums.google.com/forum/#!forum/webmasters. They should be able to provide you with additional information as to how to get image thumbnails in the search results.
Additionally, you may want to check out this post where the author investigates what factors may trigger the appearance of images in mobile search results: https://blumenthals.com/blog/2018/10/03/local-mobile-image-thumbnail-serps-its-all-about-context/.