Had similar problems with much older versions of the partition - lack of expansion on game. Shot a middlin' whitetail buck in the shoulders at 150 yards and dropped him. As I walked up to him he started trying to get up (broken back) so I put another one through him from about 50 yards out. He was still trying to get up when I got a bit closer so I whacked him a third time in the neck, finishing the job. All three just penciled right through. Took the remainder of the box back to the house and shot it into a pile of sand from 40 yards. Of the 17 bullets, 12 never mushroomed at all.
I do use the 85 gr partition in one of my 243's to good success. It might be the velocity vs the mass of the target that has something to do with it in your case. No idea otherwise.
