When I change the main project mxml file, then export it, the swf doesn't change, why?

I have an existing project from years ago that I need to modify, I am using Flash Builder 4. When I export the project it creates an swf file in bin-release, then when I modify the code in the project mxml file or any supporting files and export it the swf should change but it is not. Do you have any idea why this may be happening?

Thanks