[Hummingbird Pro] Photos not showing up in Subscribe By Email digest

I believe this is a caching issue caused either by hummingbird or smush, and that changing my configuration of one of these plugins will fix it. The problem has happened before, whereby when the plugins are updated some exclusions are lost things end up getting smushed or cached that shouldn’t be. All that said, maybe the cause of the problem is entirely something else.
What is happening is that the featured images of posts aren’t showing up in the digest that is sent out to members — they are getting a digest that includes no images, except for the banner in the header. None of the featured images are presented with the summary of the relevant blog post.
Could you please walk me through the process of figuring out how to get the images to appear in the digest. They use to, but stopped at some point after one of the plugin updates.