It is possible that it is the shot placement, personally I have shut that same bullet over the years and tried a few others but always went back to it. I shoot it with 120 grains of Blackhorn and the damage it does is hard to believe with out seeing it. Pellets do not keep well in their original container the best way to keep them is to dehydrate them and then vacuum package them.
I would recommend granular powder and 120 grains of 777 FF or 120 grains of Blackhorn this would push your velocity up to where expansion is much more reliable.