@fae2535 No worries. It's one of those quirks of the platform - it's not obvious why it'd be that way.
Mastodon isn't "one giant thing" like Twitter. When you post, it takes time for other servers to know about it. Same for replies to posts. Not all servers know about each other, so replies from one of those places might be missing for you - unless you go to the same server as the person who's post you're looking at.
TLDR: A nerdy weirdness of the system.