An ounce of sixes properly placed will kill wild birds all day long. Lower velocity at the muzzle will usually keep patterns from being blown. It could be your patterns are tighter than you think and you didn't get as good a hit as you thought. If you pattern the gun and it is thin use 1 1/8 of sixes. If you go to #5's I'd go to 1 1/8 load around 1100 fps. Pheasants are a big target with a lot of area that doesn't produce a clean kill. They also have lots of feathers that can produce a big poof that doesn't do much.