Kenmore Elite Side by side Ice build up

My freezer gets huge ice build up in the back and bottom. It builds up so much on the bottom that I can not pull out the bottom shelf. I have defrosted the freezer and made sure that the water flows through the drain hole into the drain pan. I have moved food to ensure air circulation. Air is flowing from the blower vents in both the freezer and refridgerator. There is nothing in the owner’s manual explaining how to test the defrost cycle.