Nothing wrong with clear pigeon, as others have said its the person behind the gun. Needs to take account of cartridge speed. I know from experience that the speed of the cartridge mades a difference, had fired 50 shots one day with a good kill ratio when I switched cartridges, next ten shots I hit nothing, started given the pigeons another 12 to 18 inches of lead and hey presto the birds were hit full pattern.
Best killing cartridge for 20 bore had to be 28g wincester AA+ 7.5 shot until they discontinued them.