WordPress Comments only showing on latest post... despite lots on other posts?

Annoyingly, the comments now only appear on the latest post, and no others. I can view the comments via the admin section of WordPress, but not frontend on the site.

I have deactivated plugins, and reapplied etc - to no avail.

I then deactivate my theme, and..... they reappear.

The theme support team are saying "remove plugins" and don't know what else to recommend. The theme is KLEO (legit, purchased from ThemeForest and I have a purchase code).

Any ideas what could be going on? Site is https://letsplayagame.co.uk