Since you are a resource for customers and it seems you are doing all the right things in terms of advertising maybe you should look into doing a survey of your potential customers to get a better understanding of how they find contractors for the work they need done.

It may be that you are not hitting them where they live or hurt. It could be that you are advertising in the wrong publications or direct mail is not the way to reach them. Maybe doing a seminar on what you should know before hiring a contractor would be a good way to showcase your business. This can also be done as an article for local publications (this is public relations). Are you leveraging your contractors as spokespeople for your business? They have clients who need work done that they may not be qualified to do. They should be referring people to your site for help in locating other contractors.

I assume you make your money from the contractors who are listed on your site. Maybe offer current contractors a discount on cost of subscription for referrals.

I hope these suggestions help.