Mastodon only fetches posts in certain circumstances:
https://fedi.tips/which-posts-and-accounts-can-i-see-from-my-server/
Mastodon does not currently "backfill", which can cause problems.
For example if there's an account on another server, and you're the first person on your server to follow it, and no one on your server has ever interacted with it or seen its posts shared, it may appear totally blank.
The same thing may happen in conversations with missing replies, if they are not replying to you directly.
@homegrown Gotcha, thank you! That's interesting.