Refrigerator section not cold enough

Question

Freezer section is at -15deg C but the refrigerator section air temp is at +12deg C. I have checked the fan is working (had ice buildup around it) and cleaned condensor behind the panel in the freezer. The thermostat is kicking in and out and the defrost timer is working. The air temp right at the outlet at the control box in the refrigerator section is at 1-2deg C. Have also tried setting the refigerator control to "colder" thus running the compressor longer to no avail. Can someone suggest what is left to check.
Thanks in advance.
Raoul