My site is in Persian. I won't get any results until I lower the threshold to 1, and the results are all in the same category (the result thresold is just 1). I used this plugin on 3 sites of mine and the results are all the same. I think that YARPP doesn't see posts that use unicode (UTF-8 in my case) and btw just find the related posts by category. I hadn't any problem with YARPP nearly 6 months ago (wordpress version was below 3 and I can't remember the version of YARPP).
Any help would be really appreciated.