Ice build up in my frost free upright freezer

Question

I have a Kenmore upright frost free freezer. It is approx. 5 years old. The door seems to seal Ok, and I don't have any problem freezing the food, however, on the bottom of the unit, ice builds up in the middle underneath the basket. We've cleaned it our several times, and it continues to build up ice in that one spot only. The basket actually gets stuck to the ice it is so thick.We've checked to make sure nothing was leaking to cause water build up. The door seems to still seal fine. It only appears in this one section.