For technical questions regarding the Lemmy software itself, you can check out c/lemmy_support@lemmy.ml or whatever support community your own server provides. I'm no authority on Lemmy by any means, I just set up a server of my own and did some digging troubleshooting the issues I ran into.

Browsing a community should work just fine as long as federation is enabled: the all/local/subscribed selection only matters when browsing for communities or on the combined post listings like the homepage. There are, however, a number of technical problems or policy reasons why content doesn't spread itself completely across the network.

For one, your server (sh.itjust.works) has been defederated from some sites (Beehaw, Pawb, probably smaller servers too) due to moderation concerns, either because of the open policies or because of disagreements over what constitutes as acceptable behaviour. Servers like lemmygrad.ml have been banned from many if not most Lemmy instances not necessarily because of a lack of moderators, but because what the mods over there consider acceptable is considered unacceptable by the mods of other servers.

When defederation between two servers happens, both servers start having their own view of what's in a community; sh.itjust.works/c/example@beehaw.org and beehaw.org/c/example each start having separate posts and separate comments, as if there's some kind of network problem.

As for the network problem: Kbin had some pretty severe overload issues when waves of Reddit users joined the Fediverse, and they had to put up stricter Cloudflare load balancing for a while. Part of the Cloudflare solution is to add CAPTCHAs for random web pages, which federating web services obviously can't solve. As a result, these types of downtime mean that posts don't get synchronized for a while.

It's also possible that you're not seeing certain posts because servers are overloaded. Lemmy.world and Fedia have rapidly added more hardware to compensate for the load, and servers like Lemmy.ml that didn't respond as fast have had serious downtime issues. When a server is unreliable, posts and comments don't exchange well; it's not just your frontend that's showing error messages. Lemmy.world is currently running beta software with some fixes applied and that seems to have stabilized the platform significantly according to the comments on their instance.

Missing content due to technical issues should be retried automatically, unless the retry system has been explicitly disabled by the server admin (I think lemmy.world did this, which means failed federation coming from .world may not appear). It can take a few minutes or hours, but the content should appear eventually.

Mismatches caused by defederation should start resolving themselves once federation is enabled again (for instance, after enough moderators are appointed to control the massive influx of Reddit refugees. The communities should start syncing up again and posts will be visible on both sides once more. Recent posts should start appearing on both sides, older posts will remain on one instance alone because syncing everything would quickly overload the servers.

Lastly, it's possible for a server to ban a user from another server, which will also cause desynchronisation. This basically hides a problematic user from an instance without affecting the user on their home server in any way; it's a solution to the fact that every server has their own rules and moderator opinions. If @satanlover666@example.com gets banned from sh.itjust.works, they can still post on all other servers. That means their comments and posts would show up on lemmy.ml/c/memes and beehaw.org/c/memes@lemmy.ml, but not sh.itjust.works/c/memes@lemmy.ml. Of course, when their home server bans them, that changes things.

These issues weren't present in the earlier stages of Lemmy/Kbin development, when there were only a few hundred posts to deal with. I remember joining Lemmy last month and not seeing any posts with more than six or seven upvotes because there just weren't that many servers online. The network needs time to stabilize, the Lemmy software needs to get better, and the system needs more moderation tools.

Mastodon suffered from the same problems, and defederation happened a lot more often until "silencing" (not featuring posts from particular servers but still allowing users to explicitly subscribe to other servers) became a feature. Many Mastodon servers even blocked the larger instances like mastodon.social because the moderation workload from these servers was just too much to handle, but most of them stopped doing that.

There are Mastodon servers almost everyone defederates (for example, servers featuring various forms of illegal pornography and extreme violence on their front page, or the alt-right/anarchist extremist servers that call out for violence) and Lemmy isn't any different. I doubt lemmygrad and explodingheads will ever federate with most servers.

If you don't like defederation, you can run your own server, or pick another one that isn't defederated yet. However, every platform is free to choose who they do or do not accept, and if you or your server are ruled to be too problematic, you can still be blocked. I doubt any servers will defederate you for simply interacting with servers they've blocked, but if you repeat the problematic sentiments those servers are blocked for on other servers, expect to be banned.

I'm sure Lemmy/Kbin/etc will figure this stuff out in a while. The servers, tooling, and communities just need some time to adjust.