Aneel makes some good points. Most PR firms serve large companies, who can afford to wait for reporters to come to them. Startups need an active outbound strategy that's focused on reporter needs, and those needs revolve around satisfying their fact-checking editors and readers who want a good story.
I was a reporter for years, and I used to talk and write about it:
https://drive.google.com/file/d/0BzcEIIxKcYncbUk2ZjRHS1hmQUk...
https://drive.google.com/file/d/0BzcEIIxKcYncOHd4VTBrU29ObFE...