Slide In displaying most recent posts instead of related posts.

I have created a new slide that I want to display on all posts. This slide should display one other post that is related to the post that is currently being viewed. However, the Slide In is only displaying the most recent post. So, when I'm on post A (the most recent post) when I trigger the Slide In it shows me 1 related post, which is post A, the post that I'm already on. In addition, for all other posts the only related post that ever shows up is post A. Is there a way to fix that?