The vast majority of the time, difficulties in making an edge sufficiently sharp in the first place are due to any one, or a combination of, three possible issues:
- The existing edge angle is relatively wide/obtuse - this is usually the primary problem.
- Inconsistent angle control, resulting in a rounded edge.
- Stopping short of a full apex, as would be indicated by forming and 'flipping' a full-length burr along the edge. If a burr is never formed or verified, chances are the edge isn't fully apexed.
If one has experience and good success in sharpening other knives (implying angle control is good enough and diligence is taken to form & verify a burr), but then has difficulty with only a few others, I almost ALWAYS suspect most of the issues are due to an overly thick (obtuse) edge. With my own knives, thinning them out at the edge will fix about 99% of problems like this. Even cheap knives respond much better when adequately thinned to edge angles at/below 30° inclusive (15° per side). Any wider than that, the apex will need to be almost perfectly crisp for it to cut well. If it isn't, the wider geometry behind it will really impede cutting performance.
S30V, in particular, has a reputation for giving up it's hair-popping edge relatively quickly, leaving a coarser 'utility sharp' edge behind that's more durable, but won't do really fine shaving-type cutting as well. Doesn't always behave this way, but people notice it more often with this steel. So how do you work around that? By thinning the geometry behind it, which reduces the urgency to keep the apex so perfectly crisp. At narrower edge angles, there's less of a premium on the apex itself, because the steel behind it will still be thin enough to permit decent cutting performance after the apex dulls a little bit. You'll notice this thinning benefit more in sharpness-testing by cutting paper, for example, which will remain more effortless at edge angles at/below 30° inclusive. And if you take it down to maybe ~25° inclusive or so, you'll even begin to see shaving sharpness remain a good while longer as well. Point being, the narrower angle at the edge will increasingly render a little dulling at the apex less important, as that edge angle is reduced.
There's some small possibility that some steel near the edge is heat-damaged, as mentioned earlier. The usual 'fix' for that is to grind that weakened steel off the apex; and that's most quickly done by
lightly drawing the apex across the stone, as if attempting to slice the stone in half, before starting the regrinding of the edge bevels.
