Your home instance will only start receiving posts from remote communities from the time that the first user subscribes to it onwards. So assuming you are the first person from the SDF.org Lemmy instance to sub to that community, the SDF.org instance will not backfill posts or comments from prior to the time that you subbed. If you know the URL of a specific post or comment, you can force the SDF.org instance to fetch it by plugging the post or comment's direct URL into the search bar.

See the Lemmy federation docs