My Amana electric furnace works well with a outside temp of 30 however when the temp falls below 30 it does not keep up with the inside temp falling as much as 10 degrees below thermostat setting. This furnace is only 5 years old. What could the problem be?

You may have multiple heater banks, and not all are functioning. Could be a faulty sequencer (heat relay), limit control, or an open fusible link.