To me, there are several reasons why I didn't sell many eBooks, other than my science fiction series, Forgestriker, the opening short story in the seven story selection is the image on the post.
One is my award wining selection annoyed several writers I used to chat to in the USA. There problem is jealousy, for months, I had to read of their successes, book tours, contracts, signings, you get the idea; then with no qualifications, other than the ability to write an engrossing story, I was treated like a pariah, some even trashed my work online.
I also think a large reason is I never followed a trend, I wanted to be known for my work, not as a writer who joined the trends. The only time I did write for a trend was in my Amazon bestseller - Holding Richmond - this is a vampire story written at the beginning of the Twight Saga series. The difference is my vampires hark back to the 1920's film from Germany, Nosferatu, if you are expecting sexy, young things, this is NOT your book.
There are two other reason why I struggled, one is I live in the UK, as soon as an agent sees that, I can forget my work going further than the trash pile, the other reason is I lack the funds to promote my work as it should be done.