Since the posts were already created before it was federated, other instances won't see those existing posts. You will have to manually search the URL to make it appear in your current instance.
The #ActivityPub protocol is based on when an instance discovered another instance (or an account in another instance for that matter).
Another way is to reshare that content.
Think of a timeline. Your site is technically a separate instance, that's Instance A. Instance B only discovered Instance A on 2023-10-31. Instance A will only pull/receive content published after it discovered Instance A (or account).
Instance C only found your Instance A on 2023-11-07. Instance C will only pull/receive content published after it discovered Instance A.
Note: I'm oversimplifying.