I hate to disappoint, but free books don't lead to sales.
People just load up their reading devices with free books in the hope that they
might get around to reading them one day, and never do. I have done the same
with paid books because I found them being sold cheaply. Do I buy another book,
having read and enjoyed one by a particular author? No. Why not? Because there
are simply so many books available, so easily and so cheaply; and that is even
before you start downloading free books. No doubt there are many people who do
not behave this way. If someone follows you as an author then the chances of
making another sale are good because Amazon will send them an email when you
publish new work.
Just because a book is available free doesn't mean that
people will download it. Once I thought I would download some free books so I
went onto Amazon, found a category I was interested in, set the criteria to
price from lowest to highest, so that free books showed up first. I found that
I didn't really want to download any.
If people talk about your books that is how you will get your
free books downloaded, or make a sale on your paid books. Word of mouth is the
best sales driver there is.
This might sound like a silly question but have you checked
that your free books are actually available on Amazon?