Dryer/Exhaust vents

Many of the outside dryer/exhaust vents are severely plugged with dust and lint. The screens that cover the air outflow need cleaning. The last few times that the Air ducts cleaners came, they missed many of the vents. When is the next scheduled air duct cleaning? We need to monitor the next cleaning and make sure that all vents are cleaned properly. I clean the lint trap every time in my dryer. The lack of cleaning on the outside screens has caused a reduction in airflow and potential buildup inside the ducts. Thank you.