Do you have a list of benefits on the homepage outlining exactly why it would be better to join your site then a standard one? Are your "call to action" buttons big/different enough to make them stand out? Perhaps your main page is too cluttered and drawing potential users away from the register button.
It varies from niche to niche what converts, but what you might want to consider is some A/B or MVT testing. I'm pretty sure there are some free services out there that would allow you to do this and you can slowly tweak what's working. Also it takes time to get registrations. How long has your site been live? Can you approach some people and encourage them to beta test ( a local writing group/ local college class for short stories? ) Their word of mouth might be beneficial for you