I agree with Tom. I wouldn't think of hiring a PR agency as a "box you have to check" at the Series A stage of the company.
A lot depends on your goals: what you are trying to accomplish by hiring them?
Your comment about "increase its chances" sounds like you are of the mindset that if you increase product visibility, you will garner investor interest.
That's certainly one approach to gain investor interest but ultimately, your financial statements and prospects for growth will make the final decision.
If you engage with a PR firm, I suggest you do so on a project-by-project basis vs. giving them a monthly retainer which ONLY increases your monthly burn rate which only makes investors more nervous.
I suggest identifying stories about your product success and then finding the right PR firm which can market those stories to those media outlets that you believe would be interested.