If it is a new instance install, the data starts empty and only fills each community after the first person on that instance subscribes. There is currently no tool or protocol for backfilling older content.
If you are the first user from your instance to subscribe to a community from another instance, then historical posts and comments will not be immediately synced. This is a limitation of the Lemmy software currently. However, if old posts start getting some new activity after you've subscribed (like a new comment or edit), then that will trigger a sync and the old posts should start showing up for you as well.
Yeah I have noticed on lemmy.studio, a newer server, some of the communities from either kbin or lemmy didn’t update, but then after some time, som of them diiid update, with I think all of the old posts, and even if those old posts didn’t have comments. So maybe that’s why it happened for me but some are still unupdated