Hidden Ice build-up inside plastic bottom cover of...

Question

This ice build-up is invisible unless one unscrews and removes the bottom white plastic cover of the freezer compartment. The build-up became so bad after 15 months of use of a brand new AMCOR model 560WL top freezer refrigerator that it prevented the fan behind the rear wall of the freezer compartment from turning. The end result was a non cooling refrigerator and freezer. Removal of the ice using hot water and an ice pick enabled the fan to once again turn and the freezer and refrigerator to once again work. QUESTION: how does one prevent the invisible ice from building up?