I live in a humid climate, but after 2 years of use, my Panasonic NR-F461-AX refigerator has started to frost up, so much so I need to defrost it each month. Why the change?

It has nothing to do with living in a humid climate. It is one of these three things causing the problem:

1. Defrost heater faulty.
2. Defrost limit switch faulty.
3. Defrost timer faulty.

A repairman can diagnose and fix this problem in less than an hour provided he has the part in his truck. Fix should cost less than $100 part included.