Comparing Robb and Dany is difficult.
Robb's core followers are tied to him through familial and historical allegiance -- he is their liege lord, as was his father. In order to affirm that position he had only to demonstrate that he had what it took to carry on. In other words, he didn't have to win them, he just had to keep them. At first he made all the right moves to do that, and even to expand his influence. Lately, not so much.
Dany may have a hereditary claim to the Iron Throne, but aside from the two Westeros men nobody around her cares. She gains her influence through charisma (and dragons). Her people are loyal to her because she personifies, and demonstrates, the qualities to which people want to be loyal. She is shaping herself into the kind of leader that people love, worship, fear a bit, and will follow into hell. She may need to learn how to lead, but these inspirational qualities are inborn.
One other big difference: Robb doesn't seem very good at "the Game." Dany hasn't yet had to play, although her handling of the Unsullied owner seems to indicate that when she does eventually need to play politics for keeps, she will be formidable.