While you work hard to get the word out when you publish a book, what puts it over the top (if that's meant to happen) is serendipity. Serendipity is a funny concept, and apparently difficult to explain to some cultures.
Harry Potter? Two major counts of serendipity. The first was a publisher who had no great interest in the book being nagged by his young daughter, to whom he had given the manuscript. To appease her, he printed 250 copies and shipped them off to libraries. With no faith in the book's prospects, no marketing effort was made. The second count was that the librarians liked and recommended the book often enough that it began to gain traction, and get reviews. After that - well, you know.
If the book does well, I expect the many hours I spend promoting it will mean much less than all the factors I can't control. Mood-swings (favorable or otherwise) of the reading public, emerging news, and the financial fortunes of Amazon are out of my hands. Serendipity is difficult to explain, impossible to manufacture, and completely critical to success. There's nothing to be done but accept it.
Meanwhile - here's that video. You won't recognize it, but that's one of ABBA's most famous songs - and their final recording ever.