When I turn my thermostat to the "on" mode the heat works fine. When it is set to "auto" it does not come on. I can hear it clicking on and off as I move the lever to set the temp, but it does not actually come on. Its electric. It works fine with the AC in both "auto" and "on" mode. Any suggestions?

Additional Details
When the heat is "on" (in manual) the temp is constant. The unit is about 15 years old