Hi Sergi!
We have done several things:
1. We have seen sour settings, these are good on your site!
2. We have tested the WRI plugin with RT-Theme 18 theme to see if they are compatible or not. During the test we have used Portfolio and Product Showcase post types. We used settings that display the following relationships in both sides:
-product to showcase
-product to other product
-showcase to product
-showcase to other showcase
All way worked perfectly on our own test environment, so we can declare that RT-Theme 18 and WRI are compatible, this is good news. Find screenshots attached about our test.
3. We have seen your YARPP settings.
On YARPP settings page of your site I saw the following message, that may prevent YARPP engine to find related items:
"Consider Titles" and "Consider Bodies" are currently disabled. As a result, "Pages" may not display in the "Related Posts" sections. Show Details [+]
Please click on ‘Show details’ link on YARPP settings page for more information.
You can find a lot of topics about this YARPP / MyISAM problem in the net, e.g:
https://wordpress.org/support/topic/fulltext-index-creation-problem-consider-titles-disabled?replies=8
https://wordpress.org/support/topic/yarpps-consider-titles-and-consider-bodies-not-working?replies=26
I’m not 100% sure that this is the problem, but it may cause this behavior, so try to solve it in your database, we hope this will be a solution for the problem.
Peter