Never over stuff your freezer.

It needs proper airflow around the items to keep them cold. This is why the shelves are often made of bars of thin metal rather than solid plastic.

Plastic bags bunch together and block the airflow completely.

Remove any bags or cardboard boxes whenever possible.

Put bags on the shelf with spaces between them to allow air to flow all around it.

Never block the fan or duct/vent. If you want something to chill quickly put it near a vent.