Thank you for your question. There are many reasons that could explain why you run out of hot water: The thermostat of your new water heater may not be set at the right temperature, which is 60°C (140°F). The new tank may not be the same size as the old one. Or maybe your new water heater is simply defective, in which case we suggest you find out from the manufacturer whether it is still under warranty.
We haven’t sold water heaters for a number of years. A specialist in the field would be better placed to help you.