Sorry for your bad experience, but as stated in the FAQ the plugin cannot support all themes out of the box:
Why does the frontend still display the featured image although I added a featured video to the post?
Sadly not all themes work out of the box. Themes need to make use of WordPress’ native Post Thumbnail functionality (specifically get_the_post_thumbnail() and/or the_post_thumbnail()) – these functions are where the plugin can hook into the theme and modify what is displayed. Consider contacting the theme’s creator or modifying the theme’s sourcecode in order to add the plugin’s PHP-functions.
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This reply was modified 7 years, 7 months ago by Alex.