Featured images being set only on a few posts

Hi!
First of all thanks for the plugin, its the only one that does what I want but it still doesn’t quite do the trick.

So the plugin is only working on a few posts. I have checked my posts and they all have an image on it that’s on the media library and that is attached (in the media library it says uploaded to) said post. However, on some posts it sets the featured image and on others it doesn’t. I’m not sure what the difference between posts is as they all follow the same format, have the same type of image and HTML.

Any help would be greatly appreciated.
Thanks!

https://wordpress.org/plugins/easy-add-thumbnail/