1. Have you weighed all of the arrows with the broad heads on them?
Outer limits archery sells a kit that are washers. Each weighing a grain.
This will assure that the weight of each is the same, to help them fly the same.
2. Have you indexed each broad head?
Line up each broad head with your vanes. Every blade matching the vane.
Some will sand a little material off the insert to accomplish this. But I don't recommend this. If you ever change your broad heads, you will need to sand more material off.
I use a piece of scotch tape. I put it over the insert, then poke a hole in it. Screw the insert in it and shave off the rest of the tape. (add more tape if needed) once you have indexed your blades the spin of the arrow in flight will be enhanced. Thus helping flight.
3. Have you spine tested the arrow that flies different?
Old school people would float the shafts to see where the seam would be. Then Fletch it with the seam on the bottom.
Now a days you might get an arrow or two with the seam out of place. Thus the seam being a little stiffer and causing that arrow to fly different.
Most would re-Fletch the arrow to line the seam to the bottom.