Locate the defrost control in the refrigerator and find out how to manually adjust the timer into a defrost... This control will have a small knob that turns clockwise only or a pentip-sized button to electronically start the defrost cycle... I'm not all familiar with the newer electronic controls... With the system in defrost and the rear inside cover of freezer removed observe defrost heater for getting hot... If not, you will need a volt/ohm meter to check the heater element and the bimetal t-stat for power and continuity... If you don't know how to do this procedure then you'll need a pro to check these parts... If you can determine the element and defrost end t-stat are not defective then one can assume the timer is not timing correctly... Part numbers: Heater - 2188175 Bi-metal t-stat - 2169155 Timer - 4388932
APP should be able to provide these parts... Hope this helps or extra advice can be passed on... cheers, kooler