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
Comments from before the first user subscribes simply do not sync. An instance only gets comments made after at least one user subscribes to a community.