@lb_ @Chopperwelder there's a few reasons why this could be.
Fediverse works by follow relationships on a per instance basis.
Meaning if at least one person on instance A follows 1 person on instance B then the person on instance B's posts will show up on instance A.
However, if there's no follow relationships, the posts don't federate and they don't backfill.
Only new posts from the moment a follow relationship is established, will be federated.
Also, since Mastodon allows text posts and Pixelfed does not, outside of comments, it might look like they don't have posts because they don't have any "photo" posts.
Hope that makes sense!