Hi I'm John I will help you with the problem.
You said you defrosted the freezer were the freezer coils frosted up.
Please send me the model number so I may assist you better.
So the frost was gradually getting worse and not cooling the fridge.
This is the defrost thermostat its a common part to fail in that model , its located on the freezer coils clamped to the top of the coils , I would replace the defrost thermostat it should fix the problem.
If you require more assistance let me know please.
Great it should work for you.
Thanks and God bless..