It sounds to me you have high overall staining as opposed to proportional staining. Since you are able to see different contrast when using different amounts of C.
No, if a little bit is good a lot more is not better. So maybe you put too much pyro in solution A. One way to reduce the overall staining is to increase the sodium sulfite amount. Try increasing solution B. HP5+ is known for producing a higher overall staining than most films in pyro, so maybe your change to Bergger will help.
Another cause could be overexposing the negative. What EI are you using? Most people rate HP5+ at 160 to 200 but maybe your meter/developer/temp combination requires that you rate the film higher.