Whether or not you see a note depends on whether the relay(s) that carry the note are up and you are connected to them. So if the only relay carrying the note that you connect to is the mostr bridge and it's down you won't see it. Notes do get propagated through to other relays but it's not perfect.
Right, yeah the thing is (a) there are servers that federate with Mostr/Momostr, (b) there are servers that *don't*, but both of their notes show up on the Mostr bridge.
Now within the set of servers that federate with Mostr, *sometimes* I can actually get a reply (actually the only time in recent memory is with retro-gaiden.com, kudos to nostr:nprofile1qyt8wumn8ghj7un9d3shjtnyd968gmewwp6kytcqypekqratdqzu83u6tyjacawyg7n8hag7rhwvn3mjaw8jeumr7qkgj3ncn7q ), but sometimes the reply just seems to disappear.
I have to imagine that there is some error message or something being pinged back from the Mastodon instance saying "nope you can't post here for XYZ" reasons... it would be great to have that be relayed back to Nostr, right?
Discussion
No I'm talking about on the Mastodon side. Notes should show up there if this is working as it's supposed to, but 99% of the time they don't.
If you see a note on the fedi server and then it disappears then it may be related to moderation/ defederation on their part.
WRT Activitypub I think (though Alex may advise otherwise) it is push/ pull. When you push an event to activitypub, every server that has a subscriber (follower) to your notes will pull that event. I think, depending on the sw it may pull per follower as well. You could get overwhelmed with failure messages if they're sent back (I'm not sure if they are tbh, would have assumed they would just drop stuff).