If the compressor is running constantly but the freezer will not cool properly then you likely have a sealed system problem. A service technician would need to diagnose and repair that type of problem. Here is a link for the Sears Service website in case you need it:
Sears Home Services
If the compressor is cycling on and off, then you could have a problem with a thermistor or the control board. This unit has 4 thermistors. Two thermistors are in the fresh food compartment (refrigerator). There is one thermistor in the freezer and one in the evaporator compartment of the freezer. A problem with one of these temperature sensors could be causing your problem. I recommend that you have a service technician diagnose the unit before replacing parts if the evaporator shows a frost pattern but the compressor is cycling off too often and causing the high freezer temperature.
Your details imply that you accessed the evaporator to check the evaporator heater and bi-metal thermostat. You did not indicate that you found excessive frost. If you did find excessive frost and ice on the evaporator, then you could have a control board problem or a wiring failure if the defrost heater is not working or is not being energized to melt off the excessive frost during the automatic defrost cycle.
These tips may help you determine the problem in your refrigerator but it is likely that you will need a service technician to fix this problem. If you apparently do not have a sealed system failure and you want more help with diagnosing the problem yourself, reply with additional details (i.e. whether compressor is running, fans running, frosting on evaporator, etc.) and we will assist you further.