For example, I’m on Lemmy.sdf.org and I joined the [email protected] community. But there are many missing articles and comments. If I browse directly on Lemmy.ml for the same article there ~90% of comments are missing.
https://lemmy.ml/post/1152794 has 26 comments https://lemmy.sdf.org/post/11232 has 0 comments
There could be several reasons for this, depending on what exactly you mean.
When the first person on an instance to subscribe to a remote community does so, the local mirror of that community starts receiving posts and comments from the original. But older posts and comments are not retrieved. You’re only getting stuff going forward.
So, if you’re talking about an instance that is much younger than Lemmy.ml, it’ll have only a small fraction of the backlog.
On the other hand, if what you’re seeing is that new comments and posts aren’t reliably showing up, we’ll, that’s likely an issue of the server hosting the community being absolutely fucking destroyed by traffic right now.