Most PR firms do publicity, not PR.
Publicity is the act of getting ink. Publicity is getting unpaid media to pay attention, write you up, point to you, run a picture, make a commotion. Sometimes publicity is helpful, and good publicity is always good for your ego.
But it's not PR.
PR is the strategic crafting of your story. It's the focused examination of your interactions and tactics and products and pricing that, when combined, determine what and how people talk about you.
Regis McKenna was great at PR. Yes, he got Steve Jobs and the Mac on the cover of more than 30 magazines in the year it launched. That was just publicity. The real insight was crafting the story of the Mac (and yes, the story of Steve Jobs).
If you send out a boring press release, your publicity effort will probably fail, but your PR already has.
A publicity firm will tell you stories of how they got a client ink. A PR firm will talk about storytelling and being remarkable and spreading the word. They might even suggest you don't bother getting ink or issuing press releases.
In my experience, a few people have a publicity problem, but almost everyone has a PR problem. You need to solve that one first. And you probably won't accomplish that if you hire a publicity firm and don't even give them the freedom and access they need to work with you on your story.