Does not work!

Posts being in the same categories and having the same tags are no guarantee for being related. The plugins scans multiple factors to determine what is related, you can read more on this here: https://www.relatedpostsforwp.com/makes-post-related/ The premium version allows you to change the ‘weight’ of the factors used by the plugins, so you could make categories and tags more important.

The Multisite feature in Premium means the plugin works with WordPress Multisite.

Even the titles are identical!
Even if there are other factors for relations; but, at least two posts with the same tags and categories must become related when there isn’t any other posts more related to the post. Actually there is no post related to any other post. (if there was a page with the list of related posts, available it was easier to manage the realtions)

I know multisite. I want to know that when you site activate (not network activate) it in a multisite, what difference there with the single site?

That indeed sounds like they should be related! It’s really hard to debug it remote because of the many factors tho. The related posts are manageable via a metabox when you edit the post.

The only difference in multisite is that the installer runs per site instance and not on the multisite control backend.

So you are sure that this isn’t because of the language? The language is Farsi. I can send you a back up with the posts to see.

If you could send me a xml of you content I’d be happy to take a look but I’m not sure if I would understand what suppose to be related in Farsi ?? Could you upload the XML and post the link here? I expect your content not to be private, correct?

I imported the content to a test site and the relations are made (although not as I wanted!) but this shows that the plugin doesn’t have problem. I don’t know what can cause the problem.
Is there any known conflicting plugin?

There are not that I’m aware of. Are you able to do a quick test on your website with no other plugins active?

Were you able to test this?

I have some sites with the exact set of plugins. I have tested the plugin in some of them but on one of them the plugin doesn’t work! in others it does. I just can’t understand it.

Maybe the database on the site where it’s not working is corrupt. Could you try to check the “Remove Data on Uninstall?” checkbox before you deactivate and remove the plugin (via the WordPress admin) and then reinstall the plugin.

What are the database tables?

There is the custom table rp4wp_cache and it inserts some rows in the post_meta and options tables.

Closing this due inactivity. Feel free to reopen if this is still an issue.