You have a solid premise and a compelling concept. Your characters are strong, likeable, and multifaceted. Your plot is engaging and well paced. Yet, you're still not selling your book and/or engaging readers. Why?
You're cheating, and your readers can feel it.
You may have written your book for you, but your audience isn't going to read it for you. They're going to read it for them, which means your book isn't about you at all—no matter how many months, years, and/or relationships you've poured into it. Your book is about your readers, always, and if you cheat them, they'll feel it, even if they can't put their fingers on how you're doing it.
Cheating your readers can involve:
a book cover that doesn't fit your genre;
a book blurb that misrepresents your book and/or genre;
a poorly developed character arc, especially if you skirt around the major emotional decisions your protagonist is making throughout the book;
a major but unexplained transformation in your characters;
a plot line that gives short shift to a major plot point;
a resolution that occurs behind the scenes;
unresolved scenes and/or plot lines; and/or
If you're not getting the results you want with your book, review your manuscript for these "cheats." In general, however, you're likely too close to your story and your characters to notice these types of problems. A professional editor or a solid critique partner can help you identify these and other places where the reader feels let down by your story. Fix the cheats, and watch your readership grow.