While you would first blame this issue on the hot water heater, you may find that there is hot water everywhere else in your home. If this is the case, your anti-scald setting is too high, or the shower mixer valve is faulty. If you have one, the anti-scald device limits the amount of hot water coming out to avoid being scalded when you step into the shower. Adjust this and monitor the water temperature to see if it has changed. The shower mixing valve controls the water temperature by blending both cold and hot water. It is best to leave the repair to a licensed plumber.