Freezer compartment of refrigerator not freezing

First, I think it’s important to know that the Ref side has no cooling coils, it’s basically a box and the cold air in the freezer is blown over by a fan through a channel into the Ref side. This channel has a door which regulates how much cold air goes through. You probably have two control knobs in the Ref side, one controls the temp with numbers 1-9, 9 being the coldest. This temp control tells the compressor how long to run. The other control is for that little door and it may have letters A-E, E being the coldest. If you set it on E (coldest) then this will close that little door and keep most the cold air in the freezer.
So, how can the Ref side be cold and the freezer side not freezing?

1. Coolant leak. There is enough coolant to keep the Ref side cool but not enough to freeze on the freezer side.

2. Defrost system failure. The coils on the freezer side are clogged with ice and it's just a matter of time when the Ref side won't cool either. The back wall in the freezer will be covered with ice/frost