GE sxs defrost problem

I have a GE sxs refrigerator that I have had the defrost heater, the motherboard and the defrost termination bimetal replaced.I am still getting a large amount of frost buildup on the coils which is stopping the airflow from the going into the refrigerator section. Could the defrost termination bimetal be bad and not letting the unit stay in defrost long enough? The heater is still good, I checked that myself. How do you actually put the unit in to defrost? Much appreciated if some one could help me with this problem or I will got out any buy another refrigerator. Thanks for the help, Myron McMillan. I live in Knoxville, Tn. Cell # is 865-202-6551.