Michael Barone notes that, while Republicans and Democrats both average five-point post-convention bounce, Dems have a higher degree of variation. However, the bounces can be deceptive:
Is there a correlation between the size of the bounce and the vote in November? Certainly there was for Clinton in 1992, and the no-bounce Democrats — McGovern and Kerry — both lost. But in five of the 12 races since 1964, the loser had the larger bounce.
Tom Holbrook goes into more detail:
Two things in particular seem to drive the size of the bumps. First, candidates who are running ahead of where they "should" be (based on the expected election outcome) tend to get smaller bumps, and those running behind their expected level of support get larger bumps. In this way, the conventions help bring the public closer to the expected outcome and help to make elections more predictable. A perfect example of this phenomenon is the 1964 conventions. Goldwater got a huge bump, in part because he was running 16 points behind his expected vote share, and Johnson got no bump, in part because he was running 6 points above his expected vote share prior to the Democratic convention.
John Sides adds his guess:
I would expect only small bumps for either party. Neither candidate is really polling above or below expectations at the moment. The fundamentals going into the race suggest an Obama victory, but by a relatively small margin — and that’s exactly where the polls are right now. Moreover, scheduling the convention late in the summer and back-to-back should mitigate their impact. And there is only a small number of self-described undecided voters, which may help explain why another high-profile event, the naming of Representative Paul D. Ryan as the running mate, has not really moved the national polls.