Okay, so I ran into a problem recently with the Similar Posts plugin that Rob Marsh put together. I guess he's AWOL due to health-related issues per his blog and I hope he's just not blogging as opposed to, you know, actually quite ill or something.
Similar Posts is the best plugin I've found for providing...well, what the title says: Similar Posts. Putting those related posts down at the bottom of each of my posts, both so you can find something else to read and also because it provides nice links back to other parts of the blog for search engines. Every other one I've tried just hogs the CPU every single time you turn around. But Similar Posts, with its caching bit, works fine.
I'm not sure if I just hit some threshold of post quantity or what, but it just quit on me. I could go into the database, see that all the information the plugin required was there, but still: nothing. I tinkered around with the settings for a bit but finally gave up when I couldn't seem to make any changes to my situation at all. Then, when I found that Similar Posts was still the best one of these plugins out there, I resolved to make the damn thing work. Especially after I saw one post, inexplicably, got related posts listed at the bottom when 95% the site was doing without.
All of that to come to this point: if you're having this problem, give these settings a try. After at least an hour of straight-up futzing around, I found these work. If you've got a large blog and are getting the None Found Issue, check this out. Click each thumbnail to embiggen.
I don't think Output or Placement really matters in this regard. And Filter I just have some categories (namely our Contests) removed.
Under Manage Index, Handle extended characters = No; Treat as Chinese, Korean, or Japanese? = No; Treat Related Word Variations = Fuzzily; Batch size = 100.
After many, many variations on this, this one config is the one that started my Similar Posts plugin magically working again. Hope that helps you out. Good luck.