Cold Upstairs and Warm Downstairs

I have a York Model #MA20DN21A. During this summer, the unit will kick on and blow lots of of cold air upstairs, but downstairs get very little airflow. The air that comes out of the downstairs vents is about the same temperature so I have ruled out coil freeze up. I change the filters every month on the 1st.

Could this be a clog or air restriction in the main duct that leads down to the downstairs vents?