I'm no expert, but one of the things I've noticed on razors I have great difficulty sharpening is that they have a slight warp to the blade, so that part of the blade gets sharp and other parts stay without an edge. This is very visible under magnification, if you look at the width of the edge being formed. I assume you have already checked this kind of problem, but I'll venture a suggestion here in case you haven't. It always pays to not overlook the obvious.

Another possibility is that the razor you're working on is a type of hard steel, such as "manganese" or stainless. These take a lot more strokes to accomplish an edge.

Good luck on this, and let us know what the solution was.