I replayed one game I had two stars on and got the 3rd pack after winning. I think that if you replay it and don't get 3 stars on the 2nd try the pack might be gone. All you really gotta do to prevent this is close NBA 2K14 before the game ends and it won't be marked.
For difficulty, I don't really know what it's on. All-star I guess? I haven't had any problems beating the AI teams with a 10-20 point margin. Sometimes I miss wide open layups way short and throw easy passes away. Last game I was on a fast break and made a 3 feet pass to the paint and the receiver couldn't catch it.
Easiest way to get a big margin for me is to have a 3pt shooting PF, run pick & roll with the PG and center with the PF one pass away. The AI foolishly helps on the pick very often and you get an open look from deep. They should really fix that.
In the end the rating is all about how many blocks you get, though. It doesn't matter if I win by 1 or 30, all that matters is that I have at least 10 blocks.