Forget about the lining up with fletchings thing. It's a bunch of bunk.

I suspect that the four that fly not so good do not spin perfect. I bet there is just a bit of wobble to them or the blades may be slightly bent if you have shot them a number of times..

If they all spin perfect and the blades are straight then the next step would be to move your nocking point down in very very small incraments. I'm not a believer in the thought of not having your broadheads and field points impacting the same spot and simply moving your sight.

8.3% is pretty low IMO for a hunting shaft. You may want to fletch with feathers to get the rear of the arrow lighter to increase the FOC or seriously consider a heavier broadhead as long as it won't make the spine of the arrow to weak.

You might want to post your bow, draw length, draw weight, arrow length, which arrow you are shooting, the arrows spine #, and broadhead weight.