I'm still learning the quirks of federation and the behaviors of communities hosted on different instances, so I apologize if this is something that's been asked repeatedly.
I made a post to a community I am subscribed to at lemmy.ca, from my home instance at slrpnk.net. Public posts on that community (as well as the post that I made) are not showing up from my home instance. My profile page on my home instance does not show any record of the post that I made.
I was only able to find existence of the post that I made by going to lemmy.ca, and then finding the community that I posted to from there.
I believe to ping someone, you need to put an @ symbol before their name instead of /u/. Depending on how you're interacting with Lemmy (web or app) it should provide some means of auto completing the ping.
That community only accepts posts in 'undermined' language, so if you aren't seeing anything from there, but you can when you log out (to simulate everyone else's view of it), then it's probably a user setting that prevents you from seeing stuff from that language. If you go to the 'collapse' community and posts by 'Midnight' are missing, then it'll be that (similarly there's a comment here from 'originallucifer' - if you haven't seen it, it's 'cos of the language thing).