It depends on the time and place, but often the position of executioner was a patronage position, and not a particularly desirable one, so you got people who weren’t very good at their job and weren’t hugely interested in being good at their job. For example, the infamous Jack Ketch:

Jack Ketch was Charles II’s executioner and was so astonishingly bad at his job that he became a figure of both hatred and mirth to the London public, such that to this day his name is a synonym for death, executioners, and Satan himself, and he became a bad guy in the Punch & Judy shows. (Yes, some of the research here started with Rivers of London by Ben Aaronovitch) For example, this is how Ketch carried out his job when it came to Lord William Russell:

“On that occasion, Ketch wielded the instrument of death either with such sadistically nuanced skill or with such lack of simple dexterity – nobody could tell which – that the victim suffered horrifically under blow after blow, each excruciating but not in itself lethal. Even among the bloodthirsty throngs that habitually attended English beheadings, the gory and agonizing display had created such outrage that Ketch felt moved to write and publish a pamphlet titled Apologie, in which he excused his performance with the claim that Lord Russell had failed to “dispose himself as was most suitable” and that he was therefore distracted while taking aim on his neck.”

To be more specific, he took three swings with the axe and couldn’t pull it off, and then finished the job with a saw. It takes a special kind of bastard to write, print, and distribute a pamphlet to avoid blame for botching an execution, and to hit on “the dead guy sucked at being executed” as the excuse. And this was not a one-time thing: Ketch botched the execution of the Duke of Monmouth, taking no less than five whacks of the axe to finish the job (apparently because Ketch hadn’t bothered to sharpen the axe ahead of time) and had to be hustled out of town for a while because the London mob wanted to lynch him.