The fan doesn't cause the freezer door to draw tight when it is first closed. When you close the door you trap warm air in the cabinet. The rapid cooling of this air and the air going out of the cabinet through the vent causes the door to draw up tight for a few seconds.
The reason the door isn't drawing up tight is because the air is going out somewhere else. It is usually leaking around the gasket. New gaskets sometimes take 3-4 days to form themselves to the door and cabinet. Wash the front of the cabinet where the gasket touches and the new gasket with some mild dishwashing detergent and warm water.
Close the door and lock it for 3-4 days then try it again. If it doesn't draw up tight, test the door gasket for leaks. Open the door and place a dollar bill in one corner so the door closes on the dollar when you close it. Pull the dollar out and notice if there is a slight drag as you pull it out of the gasket. Move the dollar bill over 1 width of the dollar bill and repeat the procedure. If you find a place where there is no resistance mark the spot and continue testing all the way around the freezer door. Mark each spot where the resistance was weak or nonexistent. Open the door and check the gasket at each spot you marked. If the gasket is wrinkled or flat you need to remove the gasket and soak it in a bathtub of hot water for 30 minutes. Install the gasket as quickly as possible and close the door and lock it. Check it after 2 hours and it should be ok.
There is a vent plug along the bottom edge of the door. This has a check valve in it that lets the air out of the freezer. Make sure it is in good shape.