The problem is most likely the defrost heater element(s) and/or the defrost timer. The defrost heater element(s) are located with the evaporator core behind the access panel inside of the freezer, and can be removed, tested with an ohm meter, (any reading other than infinity would most likely mean the heater element is functional) and replaced if necessary. The defrost timer can be located inside of the refrigerator, or outside of the refrigerator behind the front kick panel, or behind the rear access cover on the back of the refrigerator.
Usually if the evaporator core is freezing up and not allowing air to flow through the evaporator core, (necessary for the system to function properly) then the defrost timer is most likely working and the defrost heater element(s) would most likely be at fault.
Always place a towel on the bottom of the freezer and use a blow dryer to defrost the evaporator core and never use any sharp or hard objects to pick away the ice or the evaporator core can be damaged and the refrigerant will be leak from the system.
I have included some images to help assist you, and a new defrost heater element or defrost timer can be purchased from an appliance parts distributor.