PETER THEOBALD says:
A pigeon apparently hovering stationary over the decoys has always been a notoriously difficult bird to kill.
I think the tendency is to aim straight at the bird as though it was not moving, thereby either missing it behind or over the top.
You really must take this type of shot with a moving gun, and try to shoot instinctively, without aiming.
Or, do as I do, and simply show yourself, taking the shot as an accelerating crosser.