First of all check your RSS feed if it is delivered with the tag. To do this, go to your feed (for WordPress mostly yourdomain.com/feed) and look for the tag. It looks something like this:
<item> ... <media:content url="https://example.com/wp-content/uploads/my-awesome-image.jpg" medium="image" type="image/jpg" /> ... </item>
If in the url is empty or an image is displayed in the wrong size, it means that the desired dimension is not available. In most cases this is solved by simply regenerating the thumbnails. For regenerating the different image sizes use the plugin Regenerate Thumbnails.