At the time that I had joined this instance, I believe none of the posts from !30rock@dubvee.org were viewable on this instance. When I subscribed to the community, most posts then became available, but few-to-none of the comments/votes did. Everything since subscribing is entirely in sync. Check it out yourself; when sorting by Old, you see many posts from well before I had joined this instance, but they do not have comments and have just the OP's initial one upvote:
https://feddit.org/c/30rock@dubvee.org?dataType=Post&sort=Old
I have noticed similar behaviour with other communities, so I don't think "actions" are just stuck in the "queue" or whatever. Is there anything I can do to get my instance fully in sync with a particular community? Or could this only be remedied by an administrator of one (or both) of the instances?
Hypothetically, sure: the info exists in one place - they'd just have to arrange a way to transfer it.
Votes themselves are a simple activity, but each one can cause a surprising increase in storage requirements (every vote has a user, every user has an instance, every instance has admins, and all these entities might have avatar and banner images associated with them). Most instances are run by one person as a hobby, and paying for DB and image storage seems to be the no. 1 reason for instances folding, so there's generally a question of how much one instance should be an exact mirror of another one.