2002 Chevy Trailblazer starting problem

I have a 2002 Chevy Trailblazer that I have been having trouble starting recently. It happens about once every 2 weeks. When the engine is warm and I go to start it, it will have to crank for about 30 seconds before it will fire. My mechanic has found nothing wrong with the fuel pump or any sensors. Any thoughts? Thanks in advance.