I have a GE GSS20IEM side by side. It is not

The possibilitty is the 8 hour timer may be defective. This timerr is the one which cut off the compressor and powers the heater to defrost the coooling coils to condense the water to the try below. If this wont happen then the possibility of the cooling coils building up frost will decrease cooling. The thermostat can also be a culprit as the thermostat is also in circuit . You can see if the timer works in this cycle by shutting off the fridge for several hours and restarting to check the defrosting cycle.the timer is placed near to the compressor unit and the heater near the cooling coils. Hope you check this out. Good day .