The All timeline shows posts from communities that have at least one user from that instance subscribed to them.
If no users from instance A are subscribed to community X, this community will never show up in that instance’s All timeline.
This is because otherwise instances have no idea what communities are out there. It isn’t until a user subscribes to a remote community that the instance starts receiving posts for that community and learns that it exists.
When a user subscribes to a remote commmunity, the remote instance starts sending updates for content that’s newly created for that community and the user’s instance knows it exists and is able to display that received content in the ALL timeline.