My freezer isn't freezing.

Hi. I have a Mc Clary, Model YFL1614W-R1 . Only about 5 inches of my coils freeze over. Always in the same spot. The rest of the coils don’t freeze at all. When I turn the defrost timer off the compressor and fan shut off. The first time I did this it turned itself back on after 17 min. The second time it stayed in defrost mode for 40 min and it wouldn’t turn itself back on. What could the issue be?