Hey There,
I notice that when I clear my cache with W3 Total Cache, YARPP doesn't pick up any related posts. To fix this, I just adjust the "Relatedness" option and it "kick starts" the plugin and everything starts working.
It's annoying, but at least it is a work around.
Maybe this can be fixed in the next update :-)
Thank You!
http://wordpress.org/extend/plugins/yet-another-related-posts-plugin/