Why? Can you offer any advice, please?
One of two things could be happening here and the first is you’re misreading the target: this bird might appear to be coming straight on but it’s almost certainly losing height at the same time, and slowing down.
If you fail to take this into account then chances are you will miss over the top, or only clip it with an odd pellet or two at the bottom edge of the pattern.
To stop this happening make sure the gun muzzles are under the bird when the shot is taken.
It’s worth bearing in mind as well that a pigeon coming head-on presents very little surface area to the gun so it will receive fewer pellet strikes than it does when flying across the column of shot.
Very open chokes or a ‘gappy’ pattern can add to the problem so it might pay to try tighter chokes and see how you fare.