I had something similar happening on mastodon, I could not see posts of a specific account. And someone explained to me that at least one person / account of your instance has to follow it; otherwise the instance would not aggregate the content of this specific account. On mastodon this is sometimes pretty extreme, and plenty of comments, stars, and boosts are missing if you are a member of a smaller instance. Maybe something similar is the problem in this case. Furthermore, I guess some instances just need a bit of tune to aggregate the data, which has been produced on a different instance. The federation still has numerous issues.