Fridge and Freezer NOT cooling

My Kitchen Aid side by side fridge/freezer is not cooling. The fridge is 6 years old and I clean it out every 4 months. The fan runs constantly but there is no cooling at all. We have cleaned the coils, replaced the filter, turned the cooling controls off and then on again, checked the vents for blockage. Basically, we’ve done everything the manual says to do in the troubleshooting guide. I am willing to learn so someone please give me guidance! Thanks!