From personal experience, I have been able to get any steel shaving sharp or close enough. The problem with lesser grade steels for me is that they don't stay sharp very long with use. I would of thought 420HC would be an acceptable grade if heat treated correctly and it should take a keen edge.
I know you have mentioned that your sharpening skills are high, but as a stab in the dark from what you have stated and what I have seen with this knife, It could be that the manafacturers edge grind is quite obtuse, and when you are sharpening, you aren't finding the edge. Try the felt tip marker trick to see if this is the case. On obtusely ground edges, I normaly work one side only, on a diamond stone, until I can visually see a well formed burr running the entire opposite edge. I try to reprofile the edge to about 30 degrees inclusive or 15 degrees per side. Only once I have seen this burr, I will swap sides and do the same again till the burr forms on the other side. From then on alternate strokes per side working down to finer stones or ceramics.
If you have tried this already and not produced a satisfactory result, then maybe there is a QC issue with the blades heat treatment.