Why does the inside of my freezer frost up in the back?

Question:The inside back of my freezer builds up a layer of frost. This blocks the cold air from flowing into the lower refrigerator section. The refrigerator unit is only about 2 years old.

Answers:
Sounds as though your defroster isn't functioning. Opening & closing the freezer real often (the kids) will make it frost up faster. I had a freezer in a side by side frig do the same thing. I turned off the freezer portion. Defrosted it manually, cleaned it up & set it 1 number higher than the factory default & kept using it until. It was a Kenmore. There is a factory trouble shooting section on big name manufacturer's websites to research also.