Know Your Audience
Just because you're booking a great act and selling tickets doesn't mean people are going to buy them.
Something to keep in mind: A band that sold out a small venue two towns over may not be as popular in your area. To put on a successful show, knowing your audience -- and using that knowledge to do some precise targeting -- is essential. Who's especially popular in your area? If you're not sure, check Facebook pages to find out how many of a band's followers live nearby, or try looking into past shows to find out what sold out quickly (or at all).
A sold-out crowd is never guaranteed, but doing a little local-market research increases your chances substantially.
Next, another way to make a sold-out show more likely ...