If I sort a multicommunity by 'Hot', it will show all posts from community 1, then all posts by community 2, etc... kind of defying the purpose of a multicommunity at all. I solved it by sorting by 'New' instead, but I figured I'd mention it here because it took me a while to notice - I thought people were not making new posts at all, while in fact, it was just displaying all posts by a lowly populated community first.