Or rather, it was stuck. Carefully directed force, some thought as to how to apply the force without throwing myself out the window (which is generally considered a Bad Idea), and vast amounts of WD-40 freed up the mechanism and it now closes fully. My guess is that the weight of the window while open had shifted it slightly on the hinges - the fixed portion of the PVC hinge mechanism is merely clipped in to the runner rather than actually being screwed down, and so can move slightly.
Come to think of it, why isn't it screwed down?
Hmm... the internets suggest that there is such a thing as a "fire escape" hinge (which opens 90° to let you use it as an emergency exit), and also that there is such a thing as a "fire escape easy clean" hinge. And in the latter, the fixed portion can be unclipped to let you slide the opened window along the rail so that you can clean it. That looks like what I've got.
Well, you learn something every day.