There are quite a few factors, like how long the hub is, how much traffic it gets, the authors hubscore, and length of the hub.
Here is a link to a pretty good hub that explains all of this, including how to increase the hubscore for specific hubs.
As long as your hubs are above 40, and your author score is above 75, you are doing fine. When scores on hubs go below 40, the links become nofollow, losing their SEO benefit. And if the author is below 75 the same thing happens.