HP5 is my main film. In my experience it is not a good choice for carbon transfer work if you develop it in conventional developers. I've tried it in many developers (including pyrocat), including Rodinol. Of course you can reach a decent DR with conventional developers, by increasing development time but at the expense of higher fb+f. A high fb+f is the problem. You will have more success if you use a high contrast developer such as D-19, which can easily be mixed from scratch. I almost gave up on HP5 for carbon work until Vaughn suggested trying D-19. I dilute it 1:3. Works like a charm!
Here is a scan of a carbon print (HP5 & D-19 1:3). I scanned this before I had a decent scanner. Believe me, the print is gorgeous, even if the subject matter isn't!