Starting in 1952 the three major networks (and that is all there were) covered the conventions from gavel to gavel.
You could go to a movie theater or watch the conventions on TV. The vast majority of voters watched the conventions. Both parties got bumps from their conventions.
But in the last couple of cycles the networks have stopped covering the conventions. And tons of cable channels have never covered any of it.
So the swing voters are watching the movie channels or some channel besides the news channels or the much smaller major network coverage.
With far far fewer people watching there is bound to be a much smaller bounce. But the media lays out the expectation of a bounce, and then reports the bounce is smaller than expected. The inference is that the candidates do not appeal. The truth is he media has far less political clout than it once did. It can't create bounces as it once did.
They don't like it .. but it is true. The media has far less power to even temporarily influence voters. They don't like it and don't want to face it or admit it. But is is a fact.
I think the media has a far more insidious influence than you suggest. Big media has interposed itself between politicians and the people in virtually every aspect of the political process with media's influence increasing with level of office from local to state to federal. For example, big media will decide what, if anything, will be shown from the conventions. At press conferences big media ask all the questions with each trying so score points. At the much heralded debates, the debates are generally moderated by big media anchors or reporters with a panel of big media inquistors. It is virtually impossible for the average voter to have his uncensored questions directed at politicians and answered over the airwaves. Even when the debates employed a townhall format the questions are prescreened, and in some instances, fed to the questioner by big media.