Is your show truly relevant to your target exhibitors? Do they look to you to "make a market" for their industry? Successful shows make themselves more relevant; here are two ways to do this:
Focus your promotion on those who will actually buy the exhibitors' products or services. Identify these power buyers by name and convince a large percentage to come to the show. Then put them one-on-one with the appropriate exhibitors. The exhibitors can only access these highly qualified prospects "in one place at one time" by being in your show.
Tie exhibitors to your show by providing a unique lead service. Offer to follow up on and prequalify the leads they get at your show. You are uniquely qualified to do this. Charge full price for this service or discount it depending on how good a customer the exhibitor is.
The objective is to be a true industry-wide marketplace for your exhibitors. They think only of Widget Expo when they are choosing (one!) face-to-face, widget-industry event to be at each year.