Hi. I tried your plugin a while ago in my staging site. Today I went in to install it in my live site. I noticed that the plugin has been updated and I manage to set it up again. The only thing that didn't worked was the Default Thumbnail not showing for those post that don't have a featured image set.
Example not showing default thumbnail: https://teachers.speakinglatino.com/download/possessive-and-demonstrative-adjectives-position-of-adjectives/
Example showing featured images: https://teachers.speakinglatino.com/download/google-translate-lesson-plan/
Welcome to our forum.
Do you use any 3rd party plugin for media management like jetpack? i can see image source is showing empty,
Also, try changing different thumbnail sizes may help you and make sure you set a thumbnail Please see the screenshot