You wrote a book. You did everything you could to build your platform. You have gotten reviews for it but sales are still down.
Did you write a series?
Did you know that writing a series is a sure fire way of getting your name noticed? Seriously, I have seen awful authors get plenty of notice all because they have a series. One of the things Mark Corker from Smashwords tells is to write a series.
The more books you have out, the more your name is out there.
It is and will always be a good marketing strategy.