Fantail,

Could be the gun, could be you. Lots of good suggestions here, let me add one more.

I missed a bird 4 years ago and I am 99.99% sure its because I picked up my head to 'peak'.

I now have a mantra that I go through right when I'm about to shoot. Believe me, the heart is still pumping and the adrenaline is flowing like crazy but right before I'm about to shoot I take 1 more second to ask myself if my head is down. I even practice it prior to the season just trying to visualize a bird walking into the site and taking that pause. its worked very well for me.

If the gun is sited in like you say and you are shooting two-feeet over then you are likely moving right before the shot to 'see what happens'

Practice makes perfect...................