- Federation only sets up a channel to exchange data, very little comes across during initial federation.
- Subscription only requests that FUTURE posts/comments/votes/updates get sent to your server. It doesn't do historical backfill.
If you sub to active communities, new posts will populate in a normal way. You'll have an incomplete view of existing megaposts that predate your subscription. Click the fedi-link to scope those out on the server that hosts the community. You won't be able to vote/reply, but you can see what's there.
Within a few days, the vast majority of posts will be fully fleshed out. Try browsing by new in the meantime, you'll see more posts that we created after your subscription landed, and spend less time in ghost-town posts where you're missing all the comments.