I actually don't think her stage presence is her problem.
She is very bright. She is always well-rehearsed - maybe a bit too rehearsed. But, she has no ability to relate to people.
I met Bill Clinton once at a campaign stop in Macon GA in 1992. We had a brief conversation, but what still sticks in my mind is watching him work a crowd, and his ability to speak with (not down) to everyone he came in contact with - he could relate to everyone.
Obama strikes me as having that same ability - W, also had that kind of relatability - the infamous who would you like to have a beer with test.
Hillary is not someone who I would want to sit down and have a beer with, or even chat with - and she strikes me as someone who does not want to have a beer (or wine) or chat with anyone - unless there is money involved. Fundamentally, that is her problem. She is not likable "enough". And, for better or worse, most presidential campaigns are no different than class president elections - who do you like more? Clinton does not have enough people who like her, and there are too many people who don't like her for her to win a national election.