You know your gun better than I do, but I would be aiming 1/3 to 1/2 up, not 2/3 up as you say you did. The gun hits dead on at 90 paces after all.
I suspect a high hit above the spine, especially if shooting from a firm/hard rest. This kind of hit drops them, but they get up within a few seconds and run off, usually never to be found. Most survive.
I would certainly give it a look though.