Sounds to me you?re failing to take into account that this slow moving bird is actually dropping as it approaches the decoys and you are shooting over the top, clipping those you do hit with the thinner bottom edge of the shot pattern.
Next time try maintaining a conscious gap between the bird and the gun muzzle just as you squeeze the trigger.
And another thing, does your gun have open chokes?
With a head-on bird that?s almost static when you pull the trigger there?s always a chance it might escape through a gap in the pattern or not be hit by a sufficient number of pellets to make a clean kill.
A head-on bird showing just its forehead and leading wing edges represents a very small target indeed.
Tighter chokes and tighter patterns will considerably improve your kill rate.