No Related Posts ever available

I’ve got a couple of sites, using Related Posts, where there are never any related posts. I’m a bit baffled as to why that is.

(The sites are test sites, not yet intended for public consumption, which is why I’m not naming them here. Normally our test sites are on internal IPs and can’t use Jetpack anyway…)

Looking at the Jetpack Debugger page, it says “Everything is A-OK” and that the site’s posts are indexed. But when I look at the JSON (by adding ?relatedposts=1 to a page), I always get an empty list. Specifically, the content of the page is:
{"version":"20150408","show_thumbnails":false,"items":[]}

From trolling other support threads, I’ve tried requesting re-indexes (multiple times); I’ve disconnected and reconnected the site; I’ve enabled the JSON API. We do use a front-end cache (a cluster of Varnish servers), which I dutifully clear before and after every change, and a lot of times in between, even though it looks like the ?relatedposts pages have cache-busting headers. None of these seem to help.

One of my devs has a theory that the problem is SSL-related, based on this blog post from earlier this year. I’m not sure I buy that theory, though, because if that were the case I’d expect that you wouldn’t be able to index the site in the first place.

I know that not naming the site will make it more-or-less impossible for you to troubleshoot directly, but do you at least have any suggestions on other things I should look at?

Thanks!

https://wordpress.org/plugins/jetpack/