Need more than one product in a category to work?

I couldn’t get Related Products to show up on the product page, either with or without the plugin. I had no problem on another site. Mucking around it turns out that I need more than one product in a category for them to show up. This seems a very weird behavior and since I don’t have more than one product in a category do I need to invent them and put things in multiple categories just to get it to work?

Seems really odd….