water heater problem
How fast are you drawing out the water? At 3GPM, 6 gallons is gone in 2 minutes. It takes the better part of an hour for most of these units to recover. Your shower head should have a flow restrictor in itthat holds back to a gallon or 2 per minute.
Also, there should be a thermostat on the heater, check the owner's manual. If it is set too cool, you will draw straight hot water for bathing and such. If it is set hotter, you can mix in more cold, but BE CAREFUL! Water above 125 degrees can scald you.