If one of them is not registering (the left one seems to not work or heat up), would the coils need replaced or should I look at something else before that?

I have a E2 series Nordyne/intertherm (Model: E2EH-012HA) (Serial: E2E9711-04142). It does not heat well when it gets below 40 degrees outside. It tends to run for a long time and does not heat up to what the thermostat is set. What would cause that?