I have found that mini loop does not seem to refresh thumbnails after the loop has been generated once.
I am using Mini Loop to build a directory. When I update the thumbnail/featured image for the Custom Post Type, changes do not flow over to the Mini Loop.
I don't have caching enabled.
Here are the shortcode and ml_format I am using.
[miniloop post_type="employee" tax="departments=327" number_posts=0][ml_format][/miniloop]
<li class="directory_link_list"> [ml_image from=thumb class=list-image]<h2>[ml_title]</h2> <h4 class="jobtitle">[ml_field name=_location]</h4>[ml_field name=_dresscode]</li>