Here's my thinking: the audiences for the conventions, while much larger than we've seen for any other event thus far in the election cycle, are still dominated by partisans. That is, the viewers are very unlikely to change their vote preferences because of rhetoric or performance. In order for a bounce to occur, the message of what happened at the convention has to get out to people who didn't watch the event live. This transmission occurs via TV and newspaper coverage, but also via water cooler talk the next day.
So what was everyone talking about the day after Romney's nomination acceptance speech? Not his speech, to be sure. They were talking about Eastwood's bit of performance art. The final day of a convention is typically all about the presidential nominee. While Romney's speech may have not been the strongest speech ever delivered by a nominee, it certainly portrayed him in a flattering light, and some discussion of that the next day might have done him some good. Instead, it left him bounceless.
|"I'll bet you had a nice view of the convention|
from behind your desk, Chief."