During an asthma attack the submucosal glands and the goblet cells start making much more mucous than normal, and the mucous is also thicker than normal. This makes it very hard for the cilia to do their job, and bring the mucous up out of the airways. So now there is too much mucous being made, and not enough being brought up by the cilia. The airways are already to narrow to breathe properly because of the tissue swelling caused by inflammation and the constriction caused by the bronchospasms, so the extra mucous blocks the airway even more. This makes breathing very difficult. In fatal asthma attacks the airways can become so constricted and/or plugged with mucous that no air can get through at all.