Thanks for your advice everyone.

I swiss-cheesed a bit of scrap pine today to work out what was going on. Turns out that, for this particular set of holdfasts, my 4in apron is too deep. Optimum thickness seems to be about 2-2.5in, where they grip really well. Thicker than about 3in they don’t hold at all. I’ve drilled out one old hole oversize and let in a 2in oak block through which is bored the right sized hole, and it works really well.

Having cleaned the shanks off to eliminate contamination as the problem, my best guess is that the comparatively lightweight shank (10mm) flexes too much for the deeper holes, though I will admit to having limited understanding of the mechanics involved.