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.