If you are shooting stocked birds at normal distances and they are running off, it’s not the pellets.

An ounce of sixes is plenty of pellets for density, and plenty of energy for penetration.

You can pump the pellets up to 1200 if you like, and see if there’s a difference.

I would probably investigate via necropsy, skin a bird, and see where I hit it.

If the holes in the bird make no sense, you might pattern that load/shotgun combination. It might not be hitting where you’re looking.