Whirlpool GI7FVCXWA00 French Door Refrigerator not cooling to temp

[studioj]

Hi I have a 27cu ft Whirlpool GI7FVCXWA00 that is about 7 years old. This is one of those units where the compressor winds up and can sound like the turbo on a semi truck... when operating properly. The freezer is cooling fine. However, the fridge is only cooling to 55 degrees in spite of being set to 37. I also don't observe hearing the compressor winding up like it used to (the compressor noise is something owners frequently complain about on this model so it is an obvious sound that the fridge makes under normal operation). Setting the fridge temp lower does not seem to help much. I ran the diags on the compressor, fans and thermostats and all check out. Based on what I know of how most refirgerators work - that is a fan circulates air from the freezer box to the fridge compartment to achieve the desired temp - I suspect that the passage between freezer and fridge is blocked but don't know where either the damper or the passage are located on this unit. is that the likely cause? If so where are the cooling ducts located? 

[Hiroshi]

the airway is at the dead center of the unit directly behind the grille in the freezer compartment, you may have a frost blockage there or the damper door could be damaged... Is there a layer of frost growing on the back wall of the freezer?