Hi Chris,
By default, the plugin parses the title, content and excerpt fields and creates a keyword list for the keywords with the most occurences within these fields. Then it constructs a query against other posts to see if they match these keywords.
I suggest trying to enable this option here: https://i.imgur.com/3CHB7mV.png
This will increase the relevance of posts with the same tags as the current one to appear within the related posts list.
