Remove the two covers one high and one low and set the temp higher. This time of year with colder water comming in the house the water heater has to work harder to raise the temp of the incomming water. In the summer the incommong water is much warmer. 40 gal is a small water heater and you will only get about 15 gal of hot water with the ice cold water entering the water heater as you use the hot water. You can get a bigger water heater. even better is to go tankless and a have never ending hot water and save money at the same time.