As was mentioned you need to help the stack overcome the stack effect of the house. You can do this one of two ways, build the chimney to the proper height or allow more air in the lowest portion of the home. Ideally, of course, the stack (especially being an exterior stack) would have been built to the proper height and you wouldn't have to worry about this. It needs to be higher than the highest interior portion of the home and should be higher than the highest portion of the roofline. Many masons can lay up block and brick well, some even know "code" but few know how to build a proper chimney.