This probably won't come as any surprise, but it turns out that media outlets pay a lot of attention when a shiny new scandal erupts, but a whole lot less attention when the scandal turns out to be a nothingburger. As an example, Brendan Nyhan presents this chart showing coverage of the IRS scandal:
As Nyhan says, this is sort of a subset of the general tendency of the media to pay lots of attention to a new event when it first erupts, and then less as time goes by. This is perfectly normal: public interest in something like the Sandy Hook massacre or the Boston bombings only lasts so long, after all. Eventually there's nothing new to report and we all move on.
But in the case of an event like the IRS scandal, this can leave the public with a serious misimpression. Everyone heard about it when it first happened, but a month later hardly anyone heard about the new revelations that turned it into a non-scandal. This means that an awful lot of people are left thinking it was a big scandal even though it wasn't.
I think the most interesting followup question about this is whether this is just normal behavior or whether it's driven by the fact that conservative media promoted the original story heavily but didn't promote the followups. Perhaps a bit of both. Stories don't last forever, and the followup coverage tends to be a lot less exciting than the original allegations. So diminishing attention is hardly surprising. At the same time, it's hardly plausible that coverage of the followup stories would have been so meager if the Fox/Drudge/Limbaugh axis had been promoting them heavily. Try as it might, the liberal media still doesn't have the same kind of clout when it comes to driving media narratives.