I remember when Obama gave an indifferent performance in the first debate with Romney. He recovered quickly and convincingly for the final two and Romney supporters put away their party hats.
Kerry made Dubya look silly, the first debate too.
I'm convinced that incumbents are just not used to hard questions over their 4 years as president, press conferences don't count, and debate prep doesn't get their attention until they get their asses handed to them in a first debate.