feddit.online

tl;dr - You can force-federate older posts and comments. See steps at bottom of long post ahead.

As Jerry mentioned, when a new remote community is added by PieFed (more or less the same in lemmy too), it will try to fetch the most recent posts. If the remote community is also on a PieFed instance, it should be able to grab the most recent 50 posts. If it is on a lemmy instance, I think lemmy only provides the most recent 20.

That is how it is supposed to work. However, that initial retrieval process can be kind of buggy and not always work right, especially when crossing between lemmy and PieFed (I haven't personally worked on this section of the code at all, just my experience). Also, that process does not retrieve any old comments or votes (so, all the scores will just start at 1).

After a community has been subscribed to (in this case, by you), your instance will get each new activity in the community from that point forward (posts, comments, votes, etc.). So, from this point forward, you should be all set.

If you want to force-federate over old content that existed prior to your subscription to the community, it is possible. If you go to the community's page on your instance, in the sidebar, you will see a link that says Retrieve a post from the original server. If you click that, it brings you to a form where you can pop in a post's or comment's url.

To get the proper url for a post/comment, in lemmy, you need to make sure that you are using the link that the fediverse icon points to on that post/comment. This little symbol is what you are looking for in the lemmy ui:

Copy that link from the post/comment on lemmy and then paste it into the form on feddit and it should go fetch the post/comment for you and backfill it. A caveat is that votes are not fetched, so scores will just be at 1. Also, fetching a comment will fail if it is a response to another comment that hasn't been fetched yet.