It's your pellets most likely. Buy loose powder. It won;t grab moisture anywhere like the pellets do. Plus you can tweak your load in 5 / 10 grain increments. You'd be surprised what 5 or 10 grains can do to shot groups on paper. Sometimes, that's all it takes, along with uncontaminated powder.

One last thing! Those 777 primers are pretty weak. Consider changing to Winchester 209 primers.