Hey, thanks for the kind words, we appreciate it, really!
We noticed that you already sent the same message on our other plugin's page, over here: click!
Hope you don't mind, but we would rather have this conversation over here, since this is the plugin you have issues with (unless I'm very much mistaken?).
As for your problem — I'm not exactly sure what do you have in mind when you said "I try to manually add other posts from the site (which is not what I want to do), they aren't even added either" — how do you try to add your related posts manually? Via that big orange button called 'Edit Related Posts', just below your published related posts section or via some other way?
Also — would you please be so kind and send us a screenshot of your current plugin settings, with all sub-menus expanded? And if you have any custom CSS settings implemented, do let us know which ones, since we'd love to take a closer look at it, if possible.
We are very eager to help you out, so any additional feedback is greatly appreciated! Take care & I'm looking forward to your reply!