I’ve been successfully posting in communities at lemmy.ml from lemmy.sdf.org, so I’m not sure what might be happening there. I was able to find Midwest.social/c/chicago from the sdf instance search. The URL should work, but you are also able to use the format !community@instance.tld (!chicago@midwest.social). The search does take a long time to fetch the results from an external instance, and there is no loading animation, so after a minute or two the results will change from “no results found” to displaying the community.

My suspicion is that because federation takes time to propagate, posting might seem "slow" for a while until the communities become more established.

Patience is key here, and with more use it'll get better!

There's no blocking. Lemmy's just a little bad at this right now.

If nobody on the instance is subscribed to a community, it's going to basically be invisible. To reveal it, you need to manually try to navigate to it via the search UI or a URL. The easiest way is to paste this pattern into the search field: ![communityname]@[instance] (e.g.: [!technology@lemmy.ml](/c/technology@lemmy.ml)). The community won't immediately appear when you do this, but you can come back a minute or two later and it'll suddenly be visible in the community search.

As for comments/posts not appearing on lemmy.ml, keep in mind that posts are only synced after the community is first discovered via the aforementioned process. There's no guarantee you'll ever get historic stuff to show up, only new stuff.

Both of these things are unintuitive and hopefully will improve with time. FWIW: There's already discussion about how to improve the search feature on Github (example). In the meantime, you can do your searching here and then do the search trick to pull in communities you'd like to join.