How many evaporator fans do you have? Are you sure the left hand motor is working? Might seem a silly question but sometimes they can appear to be working but are actually just being pulled round by the others. Have you measured the superheat across evaporator? What refrigerant is the system using? Is there any frost at the compressor? Are your defrost heaters pushed right through the evaporator coil? Sometimes they can shift left or right, out to one side leaving a small area top to bottom, as you have described, with no heaters. More info required.