Readers who seek free books all the time are open to many new genres, but also rather impatient when reading. This means that if your book is not interesting on the first few pages, they will delete it. As they got it for free, they do not value it. This is why it is very important that interesting events take place right at the start of the book, on the first few pages. Some people download a large number of books, knowing in advance they will only read the best ones.
You should be aware that many people do not read books immediately but download them for when they go on holiday. This means that they will read your book in a few months.
Another danger is that readers who are looking only for free books will perhaps not be ready to pay the normal price of your books, because they think there are so many free books that they cannot possibly read them all anyway.
In addition, when you are offering a book to a wider group of people, you are not actually focusing on your target audience. This may result in more negative reviews than you would otherwise get.
If you have already offered your book for free without any success, you must not despair. Maybe it is better if you find your target audience via Twitter or FB and tell them where they can get your book for free. Or send them a Smashwords coupon for a free book.