I think you will find this really cheeky, but can you make a third thread without any matching words in the thread subject and see if that works?
What I was after was that the similar threads box would show the threads with the same prefix even if the subjects had no words in common.
See, this thread
http://mybbhacks.zingaburga.com/testforu...http://mybbhacks.zingaburga.com/testforum/showthread
Has the key words Relevancy Test - but does not show with the other two as it has NO prefix - yet it is relevant. If you made a thread called green cabbage and give it prefix1 I dont think it will show in the similar threads box. I put my relevancy up to 8 - still no sign