Continue experimenting with different bullets, weights and powders. What should shoot good does not always perform. You might try some Sierra bullets.
I think your results tell you what you need to know. Some combos work better than others. Stick with the loads that perform.
As was mentioned, reading forums, people are led to believe that half minute groups are the norm and they are not. My 20 inch AR regularly shoots groups of 1.0 to 1.25 inches, range trip after range trip. I try different loads but results seem to be fairly constant. I accept that. Sometimes I will shoot a little better but nothing I can bet on.