A. Clear the drain tubes of ice or crud (drain under evap coils)
B. Closely inspect door gaskets for failure to seal, like door not closing a bit, letting air in. Or rack gearing gnashed.
C. Keep all liquid containers covered to inhibit evaporation
D. Check defrost heaters with an ohm meter, be sure they are not open circuit
E. Ensure heater element relays are actually providing voltage to the heaters.
F. Schematic hidden on unit somewhere