I think I've found what looked wrong. First, I realised why the centre of the bow seems stiff. It's curved slightly forward in the relaxed position. OK for a modern bow, but I'm not sure it would be right for a medieval bow. Though the medieval bowyers did apparently put in that little forward curve at the tip of the bow. But what makes it look wrong is the bowstring. It gets shorter as you draw the bow. By about 20% of the total length. I've measured it (use the box primitive, y-scale=400 fits along the string from bow-tip to arrow. Draw the bow, rotate to line up with the string....) It still looks a more realistic design than other bows I've seen -- there's a streak of cinematic fantasy in a lot of Poser weapons.