At the end of the story, why did the knocking stop so suddenly?
One of the key features of the plot is that it is never explicitly demonstrated that the monkey's paw has any magical powers at all. Mr. White explains this to his son Herbert early in Part II.
"Morris said the things happened so naturally," said his father, "that you might if you so wished attribute it to coincidence."
When Mr. White makes his first wish for two hundred pounds, he receives that exact amount from Maw and Meggins as compensation for his son Herbert's accidental death at the factory. This could be the magic of the monkey's paw, or it could be a sheer coincidence. We see that Herbert stayed up late the night before because they had an interesting guest in Sergeant-Major Morris and that father and son drank more whiskey than usual in keeping up with their "bibulous" guest. Herbert must have gone to work feeling hung over and without having had a full night's sleep. This could explain why he was careless and got himself caught in the textile machinery. It is significant that White's first wish was modest. He didn't ask for a million pounds or a royal palace. That would have been a real test of the monkey's supposed magic.
When Mr. White makes his second wish it is a real test of the monkey's powers, but we never know whether the wish was granted. All we know is that someone comes knocking at the door. Like Mr. and Mrs. White, the reader assumes it is Herbert, and like Mr. White, who has viewed his son's mangled body for identification purposes, the reader assumes Herbert looks like a horrible monster, especially after decaying in a grave for ten days. But the only way to find out for sure is to open the door--which is what Mrs. White is frantically trying to do.
Mr. White regrets acceding to his wife's insistence and wishing for Herbert to come back to them. The text does not specify the wording of his third wish, but the reader knows he is wishing for the knocking to stop and for the person knocking to go away forever. That person could have been Herbert. But on the other hand, it could have been some stranger who was lost out here in these dark, desolate suburbs and was only trying to get directions. We could, as Sergeant-Major Morris said, attribute the knocking to coincidence.
A clue that it might have been an innocent stranger knocking can be found in Part III.
He sat until he was chilled with the cold, glancing occasionally at the figure of the old woman peering through the window. The candle-end, which had burned below the rim of the china candlestick, was throwing pulsating shadows on the ceiling and walls, until, with a flicker larger than the rest, it expired.
The author must have written those sentences for the specific purpose of suggesting the alternative possibility that it was a stranger knocking. The light shining in the window and throwing pulsating shadows on the ceiling and walls would have attracted the attention of anybody who was out there in the dark. That would explain not only why the stranger was knocking but why he would have been knocking more and more insistently. He knew there was somebody at home, and they were his only hope in an area Mr. White had described early in Part I as
"...of all the beastly, slushy, out-of-the-way places to live in, this is the worst."
So the knocking could have stopped so suddenly because of the magical powers of the monkey's paw, or it could have stopped so suddenly because the hapless stranger gave up in disgust after realizing that the people inside were just not going to open their door. It could have been Herbert returned from the dead to move back in with his parents and fill them with horror every time they looked at him; or it could have been a coincidence that a stranger came seeking assistance shortly after that third wish had been made. The reader will never know, but he will probably believe, like Mr. and Mrs. White, that it was their son Herbert.