See on the other grills (foot/mirror), if you feel cool air from their then it means it is still leaking and the duct is not closing properly.
If you feel no air pressure, and partial cooling in the driver side grills then the symptom you are telling is due to some very minor leak in cooling coil. Check the Receiver Drier sight glass in the engine bay, if the flow is steady and constant.
It is also possible that the vent duct is not properly connected/closed behind the dashboard and fresh air might be mixing up. Also possible is that the duct behind the front grill is not connected properly, due to this the cooling might leak within the dashboard. Check if the upper or lower portions of the grills if they are not getting too much cooled.
Also check by bypassing the heater core lines, heater lines are also their at the driver side portion grills and they might also mix up with the cooling.