So I just upgraded from 3.? (not sure what I was at) to 4.1. At first I couldn't get any related posts to show anywhere (automatic display, widget, edit screen). Then I turned the match threshold down to 2 and it started working. But I noticed the match numbers for my posts where over 12, so I turned it back up to 5 and they still showed. There seems to be a bug where it has be turned down and then back up.
Just wanted to put this out there in case someone else is having a problem after updating.