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). 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.