No post thumbnails showed

Hi, it is really a great plugin, but now I’m having a problem.
I use “Ultimate Member” plugin and in the user’s profile page I have created a new tab in order to show the list of the user’s favorited posts, but post thumbnails are not showed in this list even if I have used this shortcode: [user_favorites include_thumbnails=”true”] as suggested in the plugin web page; Only post titles are showed.
Someone could help me? Thanks