'My book is no longer selling well and it's been out for 6 months/a year/two years... why not?'
Before we get to that, there are a few other things to consider.
Firstly, the marketplace is chock full of books. All are of differing quality, size and shape. As an author, you want to write a good book; a great book that will stand the test of time. and that is not easy. Writing a good book is actually difficult. Take into account wanting it to stand out above the rest, be remembered and lauded for years to come... it becomes even more challenging.
An important thing to remember is you cannot only write one book and expect to make profits from it indefinitely, if at all. Unless you end up fortunate enough to wind up with one of the big five after securing a literary agent, then you have to acknowledge that it will take a lot of work, time and effort to make your book stand out from the madding crowd.
This is of course, absolutely possible. Rachel Abbott, Mark Dawson, LJ Ross and Adam Croft are perfect examples of authors who have been extremely successful as self-published authors. It didn't happen overnight for them however; they had to spend a great deal of time and money to see a return of investment. Mark Dawson's debut novel in the John Milton series is excellent, but it didn't make him an overnight success nor did his previous published titles.
Of course, it depends if you only got into writing because you wish to make money. Correction, we would all like to be able to make a living from writing but the reality is that only about 11% of authors can and do.
The correct way to look at it is that you write because you love it and always wanted others to enjoy the stories you have in your head. If they happen to make you a few quid, then that is excellent!
Nor is it about churning out book after book necessarily. Yes, you cannot just write and release one title and rest on your laurels (notable examples of only one title being a hit or classic are Harper Lee with To Kill A Mockingbird. It would more than 35 years before she had another book published or E L James who made a few pounds here and there with the literary classic ' Fifty Shades of Grey'. Love it or hate it, it captured the zeitgeist).
You need to be releasing more books to keep your author name alive and in readers minds. Some put out three or four books a year (I'm not saying that that is the standard; I couldn't write four books a year and them be anywhere near good. Some might argue that I only release two a year and they are rubbish, but that is for another blog!); others release less. James Patterson releases 1002 titles a year and has sustained success (I'm exaggerating slightly, he only releases 964 a year), but the point is you cannot necessarily release only one book and then wonder why, 6 months to a year later it isn't doing as well.
Any publisher should continue to promote all their titles, but there will always be newer titles that take priority in regards to the scope of promotion. Here we afford all our titles promotion in a variety of guises; some with P.R support, some with book trailers, some with reductions in price, social medial advertising... but all of them are continuously promoted. After all, you can't promote every title at once, all the time. Even Peter James doesn't have Pan promoting his very first book. Readers buy his old books because they see his new releases.
The thing is, do you wish to write for the market or yourself? Personally, I think you should always write for yourself; write the books you would like to read. And if there is a market out there, eventually it will be found. Traditional publishers tend to focus on commercial viability and books that will win awards. We live in a literary climate that is perhaps too selective based upon the notion that good books are a rare commodity and only produced by notable publishers.
That is crap. So those who may choose self-publishing are advised to crank out title after title to increase visibility. If an author wishes to focus solely on Amazon rankings, how many reviews they have and the number of followers they have on social media then they are, perhaps, barking up the wrong perverbial.
Don't get me wrong, social media is an invaluable tool nowadays and you can't market or even write a book without it. And yes, Amazon is important as are other book platforms; reviews are a good sign of the quality of your work but they are not the bee all and end all.
Your story should show it has taken time, have been proof read, copy edited, formatted meticulously. And that's before you get to the cover. Create (with professional help if necessary) the most beautiful, eye catching cover that will grab attention immediately. All the the aforementioned authors in the Mark Dawson paragraph write books that are professional, expertly written, edited and have the look of a bestseller (in fact, the majority of them are, locally and internationally).
But you focus should be on writing an excellent book, one that will still be ticking over of its own according in years to come. Be proud of the book you write and you will see it on social media from time to time and on readers bookshelves.
Everything else - the Amazon sales and rankings, the Twitter sharing and Facebook plugs, admiration and awards - will all come of their own accord if your book is good enough. Not only that, it will be well deserve and hard earned. It doesn't matter if it is with Harper Collins or Amazon KDP and self-published. If you write it, they will come.
Be a wordsmith, not a bean counter as one journalist once said. There is no one saying don't write multiple books and release them; just don't feel pressured into doing so as being made to believe it is the only way to success and don't be overly concerned with the financial mandate.
Write a book that will stand the test of time or that captures reader's imaginations and sends them light years away on dangerous adventures or fills them with feelings of love and longing and the rest will come.
Trust me. I'm a nurse😉