Your home lemmy instance syncs content from a remote community after the first user on your home instance subscribes to it (except for a very small subset of posts that your homeinstance pulls on subscription). So if the post you are referring too is before your homeInstances first subscription then your sort of out of luck interacting with it. There is also a chance your homeinstance federation screwed up sometime and didn't sync something, I've seen that some (I host my own instance). You could go see the "missing" content that doesn't exist on your homeInstance on any remote instance. However, you are not going to be able to interact with it unless it exists on your home instance.