Dryer vents clog when enough lint accumulates in the system to cause a blockage. Dryer vent ducts can clog in a variety of ways. For example, a long dryer vent run will clog quicker because the lint has too far to go. It clogs toward the end of the run first, where the airflow is slower. Secondly, when there are turns or curves in the vent, lint will build up there. If the transition hose is crushed from the back of the dryer that connects to the ductwork, you get less airflow, which can also clog your vent. On the outside of the house is a termination vent. Those can get lint stuck on them which also creates a clogged dryer vent duct.