I’ll give you two links to the same post but on two different Lemmy instances:

https://geddit.social/post/17196

and

https://beehaw.org/post/659342

It’s the exact same post, with the same user who created it and everything.

But on the Geddit instance, you have several comments answering the question. On the Beehaw instance, you only see my test post (which you won’t find on the geddit’s version).

And truth be told, I have no idea why there’s a difference there. Both servers are federated with eachother:
https://beehaw.org/instances and https://geddit.social/instances
And yet, there’s a difference.

edit
One thing I could think off is that maybe it’s not federating because there are no subscribers there. I’ll try that next.