I cannot see comments made by lemmy.world instance when logged in to discuss.online. Not sure if any other instances are affected this way. I also cannot search some communities. I have not figured out a pattern. I wonder if it is intentional. Here is an example: https://discuss.online/post/202378. There are a few comments under the post, but I do not see them.
Federation of messages is limited to communities in which at least one account on a Lemmy instance has subscribed to on another instance. At that point, only new posts and comments of that community will be federated.
For example, if no one on discuss.online was subscribed to https://lemmy.world/c/gamingsuggestions at the time https://lemmy.world/post/943892 was posted, there will be no comments federated to https://discuss.online/post/202378. Once a user on discuss.online subscribes to https://lemmy.world/c/gamingsuggestions, only posts and comments from that point forward will be federated.